Fate has a much more narrative approach, less GM authority, and player-based plot control mechanisms. This can make for trouble transitioning from a more adversarial GMing environment.
Fate Core (and other recent Fate games such as Dresden) actually do a pretty good job of providing a suggested "menu" of powers and stunts for players to take; show them to your players and let those inform other stunt ideas through play, rather than trying to get the characters completely nailed down in advance the way you would have to in D&D.
To address the specific problems you mentioned:
The "blind sniper" problem is that in Fate, it's very easy for a cooperative party to stack multiple temporary aspect bonuses on a situation, then tag them all for a single super-successful roll regardless of innate skill.
Consider a sniper with a base skill of +0 - "has no idea what he's doing". First he hides on high ground (creating aspect "On a Grassy Knoll"). One of the other players - a tech - has made, and gives him some "Precision Armour Piercing Ammo", with suitable aspect. Then another player jumps into the road to stall the target into "Standing Still for a minute", so the sniper can create advantage by taking an acting to put him "Centred In My Sights".
Then the sniper free-tags all of those, for a base roll of +8 and a near-certain hit - with fate points to spare if he needs them.
Stacking enough of this sort of this can greatly reduce plausibility, but it's obvious behaviour for a group of D&D players where it's an expected part of the system to need to stack every combat advantage you can generate. I don't, however, feel it's as much of a problem as some Fate players do - this is behaves-as-designed. Fate characters are supposed to be able to beat pretty much any single obstacle if they can generate a convincing narrative. "He's not that good a shot, but it worked because an entire team was helping him take it" is a pretty good narrative to me, and gives the plot and characters room to develop further.
The "constant compels" problem arises when GM and players get caught in a Fate-point loop of constantly compelling aspects to force behaviour or non-actions from each other.
Remind your players, and remember as GM, that the point of compels is to implement narrative development. The Fate point economy will naturally limit how far players can push this, so it's not a problem unless the GM gets sucked into constantly offering Fate for compels. So don't. Compel when it's a narrative or character development to do so, not just because it's possible.
Long combats occur because groups have trouble actually taking out opponents of similar skill levels. This is particularly likely if your group is used to D&D, and trying to "wear them out" by all attacking individually. In Fate it's much more effective to set up a narrative about a couple of major attacks, taking multiple actions to support each other and make those go off well.
The best counter to most of these issues is threefold:
Let them have their moment. Players being able to tear through obstacles like this is not a problem in Fate the way it would be in D&D; it's expected behaviour of the system. Don't make it impossible, but ask what happens next and generate new obstacles. The problem can come more from a D&D-oriented GM feeling he has to make tasks "possible to fail" than from actual issues.
Your enemies are not idle. Don't GM in direct opposition to the player's actions, but instead change the game. Fate uses the same mechanics for social and combat conflicts, and it does so for good reason, expecting them to intermingle. Use it. Enemies will attack on social fronts, run away if they're in trouble, consult allies, adjust schemes, and all the other narrative options that are not often available to an orc in a dungeon.
Remember that in Fate, the player is always an informed participant, even when the character isn't. The gap between player and character knowledge is much bigger in Fate.
Situation: A hostage is tied to a chair. There is a trap which incinerates the room if anyone touches the hostage.
D&D answer: Tell the players nothing. Make hidden Perception checks. If anyone explicitly searches, make Search checks. Lie unless the players succeed at these checks.
Fate answer: Tell the players immediately that there's a trap. Let them make rolls and use aspects freely to see if the characters spot it. If one of the players has a "reckless" or "rescuer" aspect, compel it - offering a fate point if their character runs in carelessly and sets the trap off. Make the players partners in decisions that hurt their characters.
Make his allies part of his character.
This isn't always going to work, and exactly how it works best will depend on the nature of his relationship with his allies. I think this strategy is best when his allies are well-defined but aren't supposed to actually do a lot of game-changing stuff themselves: they're strong support characters, not equal party members.
Allies as aspects
Give the character sheet some additional aspect slots (on top of the usual 3 to 5 aspects Fate gives every PC) which can only be used for allies. Each ally is described by one of these character aspects, describing the player character's relationship to the ally and the ally's specialty or both:
- Sensei Ping, Master of the Wu-Han Thumb of Death
- My best friend El Sapo Dorado of the Mexican Luchadores
- The wizard Dallben, my foster father
What does this mean?
- As with any aspect, this means certain things are just true (perhaps the wizard Dallben being in the party means they can all fly whenever they like).
- Aspects provide new contexts for action. Just like The Last Kryptonian can justify Perception checks to notice something on the other side of a wall, El Sapo Dorado's aspect can justify Rapport checks with someone who only speaks Spanish.
- You can spend Fate points to get bonuses or make things be true (declare a story detail) because you have your allies with you (for a Fate point, Sensei Ping knows the ancient language of Shangri-La).
- Aspects also modify the target difficulties for rolls (El Sapo Dorado probably lowers the difficulty for breaking open doors, but increases difficulties for squeezing through ventilation shafts).
As always, these aspects imply a lot that they don't say explicitly: we're wrapping a whole person into a single aspect. If Sensei Ping is sensitive about his age, that's something everyone needs to understand is implied by the narrative, and so it can be invoked and compelled through his aspect even though the aspect doesn't say he's sensitive about his age.
What if my ally leaves?
If the absence is permanent, you can modify the aspect to reflect this (I must honor the late Sensei Ping's sacrifice), or have an empty aspect slot, until you pick up another friend and use the slot to describe your new ally.
If the absence is temporary, you modify the aspect to reflect this. Sensei Ping is conferring with the Clan of the Pointed Stick still gives you all manner of benefits because Ping is still your sensei.
Does this mean my allies can't be attacked?
Not at all! They're part of your character's fractal and so attacks against you can be narrated as targeting your allies (and may actually provide opportunities for you to be attacked when your character isn't in the room but Sensei Ping is!).
Naturally, attacks against your allies are defended by your own skill rolls, and attacks which hit your allies still inflict stress on your own stress track.
So how can my allies be hurt, stymied, or defeated?
Through consequences or the Create an Aspect action, or both.
Consequences can make allies less useful, and can create interesting plot twists. Remember, your allies are part of your character so they share your limited consequence slots.
- If Sensei Ping lost his cool (+2 consequence) that means he's no longer able to perform his most deadly moves, which require inner peace and utter self-control, until he calms down.
- When El Sapo Dorado is unmasked (+4 consequence) his shame may cripple his ability to participate for the rest of the mission. If you're playing Fate Core instead of Accelerated then the recovery action is probably retrieving his mask or killing those who saw his face.
- Dallben is kidnapped (+6 consequence) is pretty major: you lose nearly every benefit his aspect gives you, and the next part of the story is firmly dictated. Time to rescue the old man.
Your enemies can also (probably with great difficulty and usually with active opposition) Create an Aspect which hinders your allies, like Caer Dallben is under attack or La Cage De Lumiere can trap the untrappable El Sapo Dorado, or I am immune to the Wu-Han Thumb of Death.
Isn't this overcomplicated or unbalanced?
Not really. Previous Fate games like dresden-files gave players eight or ten aspects by default. I'm glad Fate reduced it to five, but players CAN handle more aspects if the aspects are backing up an interesting narrative. And it's a lot less complicated than running a handful of mooks, or full characters.
We're talking about a single-player campaign so there's no other PCs to get jealous of your bonus aspect slots. The GM has to take your extra aspects into account when creating encounters but it's not a big deal.
If you're really concerned about balance --perhaps you want to use this conceit in a multiplayer game later on-- I'd make an extra called "Allies" which costs one Refresh for every two ally-only aspect slots you get.
Best Answer
So, yes, that stunt is awesome and very strong, given the right circumstances in which it can be used. Yes, it might even be a little bit broken.
But yes there's things you can do about it to not have that be a problem, and I think there's a number of things you missed in how it gets leveraged.
I also don't think we need to play wording trickery to find ways to nerf the stunt. It's not necessary to say "five foes are just one if I count them as a group mechanically". We both know those are still five foes; that's putting mechanics first in a game that prioritizes fiction first. We also don't need to say there's a limit of three just because of the stunt's name. Both of these are sorta cheesy moves which might earn us some bad looks from players for good reason.
It'll help to recognise this stunt for what it is: it's the Swashbuckler in their element. They're about dueling multiple people at once and showing off, swinging from ropes and chandeliers like the Swashbuckler card says. This is where the character is just totally awesome and excels, and these situations are your opportunity as the GM to be a fan of them and hand them cool stuff to do. It's alright if the players wipe the floor with the opposition as long as everyone's having fun.
But that stunt's for if and when you put them in their element. If you throw nothing but hordes of enemies at this dude in environments full of useful doodads and flashy defense opportunities, yes he's going to do excellently at holding them off because he's meant to do that. Cheer them on when you do this, but don't just do this.
Let's talk about the stunt's relevant element
As you might be aware, stunts are balanced proportionate to how often they can be relevant. That's why we have a limitation for action, approach, and situation, and sometimes once-per-X restrictions and fate point costs. So let's talk about the situations this stunt has to key off that you should pay attention to.
It has to be a flashy defense: Christopher talks about various kinds of defenses here. This means doing things flashily actually has to be valuable.
It has to be using the environment. He'll be doing rope swinging, pulling own curtains and waving those around, generally just looking awesome. Some environments won't present anything much useful to the actual threat you're facing.
Those last two points considered, you should be asking your player to describe what he's doing in the fiction. Listen to his description — it should be clear that whatever they're doing is actually flashy, and that whatever flashy thing they're doing is actually a valid defense. Sometimes he'll describe something that's not really flashy at all, or just not going to help. You can call out weak descriptions and suggest it's not going to work, and offer whether your player wants to do things differently.
A flashy defence isn't going to help you much if someone you walked right up to is just dropping a grenade at your feet, for instance. That takes being quick or clever. Flashy probably doesn't help when you're in the middle of a gang of ten murlocs and it might be hard to utilise your environment from there, and it's unlikely to help when your opponents are dire wolves who just don't care about anything you could actually do in your environment.
It has to be against multiple opponents surrounding you. If they're not surrounding you, or there's one or two of them, this stunt isn't very relevant.
It has to be a (usually physical) fight against foes. That bears emphasizing all on its own. You could well have loads of fate sessions without fights happening where this stunt is relevant. Fate does very well at things other than fighting, and those are times other players can shine and the swashbuckler won't be holding down the fort with ease.
Pay attention to these limitations, and don't compose all your opposition out of textbook examples of the exact scenario the stunt was built for, and you won't see it being used constantly and looking outright insanely good all of the time.
So how do you different things and challenge the swashbuckler?
The bad guys aren't just mooks who are limited to +2/-2. They can have six approaches up to +3, you can put opponents up higher to +4 to give your players a serious challenge, you can have mooks that do more than just +2/-2. It's worth rehashing this as part of how challenging them will go.
Leverage mobs and teamwork to have your murlocs group together and act in unison to be more effective. You'll act with one murloc, with each extra murloc providing a +1 teamwork bonus to its roll — five murlocs now have a +4 bonus just from working together, versus the swashbuckler's +5 bonus to defense against them if his stunt applies. That won't be overcome so trivially, and will be just the one action for him to defend against (and possibly succeed with style on for one boost). The Fate Accelerated section on being the gamemaster points to this as an option for building sturdier groups of mooks beyond simply giving them a stress track.
(When you're not building mobs but have large numbers of enemies, you should use groups of mooks just because it's not all that interesting rolling ten different murloc attacks. Making them just two or three groups keeps the focus on your players' turns and is probably going to be more fun for everyone. It also makes for a more manageable number of actions either side might succeed with style on or need to spend resources on.)
Create advantages yourself sometimes that make it hard for him to effortlessly be flashy against everyone, like having someone toss out a smoke bomb into his vicinity. It's hard to be flashy when you're Spluttering in a smog. It's also hard to be flashy if you're Pinned down by archers or being Tied up. (All of these things can be done without having enemies present and surrounding him.)
Don't have all the enemies go after the swashbuckler and surround him. Have some go after the other characters; they've got no reason to all focus on the one guy. (People in real group fights will try to divide up their efforts, you usually can't afford to just ignore the people beating on you.) If they do focus on him, have them not always use surrounding tactics. Your swashbuckler doesn't get extra +1's just for the murlocs being in his vicinity, they actually have to be foes surrounding him in particular.
Narration that's distinctly to the player's advantage should (usually) come at a cost. You should encourage your players to help you narrate things, but when those things are distinctly to their advantage (like "that gang of murlocs is bunched up ready for me to leap right into the middle!"), that should require a cost such as a fate point or a create advantage roll. On the flipside, things they narrate that are distinctly unhelpful should probably be treated as compels if you run with them.
Speaking of compels, use those. Compel him for things to go south. When he charges in against a gang of ten goblins, offer him an event compel and say there's an ogre that was following behind them that's now showing up and he's trapped between it and those goblins. Compel that the curtain he's flinging around catches fire on a nearby candle and it's spreading to his clothes. Compel that the rope he's swinging on breaks and drops him in a terrible position. Compel him to slip off the banister he's trying to balance on and come crashing down on another player (who you also compel), putting them both at the mercy of something bad. All the ways the swashbuckler uses their environment are also ways for you to offer decision and event compels for things to go wrong. A lot of them could be quite entertaining, should the player accept and not buy them off. Don't over-compel them though to the point they feel they can't even use the stunt.
Do challenge your swashbuckler with single, strong enemies sometimes, even without a compel showing up.
Also, remember physical fights aren't always modeled with conflicts. Sometimes they're only worth a roll or two, a challenge, a contest, something like that. Your players will rarely (if ever) have an opportunity to use a defend action in those circumstances.
Don't just shut down the stunt though.
Your player's got a thing they can do well and they'd like to make use of it sometimes. Give your swashbuckler chances be awesome and flashily defend against all the things, and be a fan of them when they do so. Everything I'm saying here is just ways to not have him then go on to steamroll the entire fight and turn every opposing force into wet socks.
The Fate GM is there to also be a fan of the players, to help them be awesome, and help conspire with them against their own characters for everyone to have fun and dramatic stories. So, be supportive. If you're coming from D&D, super powerful features are often seen as a problem that needs fixing, nerfing, or to be totally stopped. Don't sweat it here — let those characters be awesome with their stuff, and give them opportunities to use it and have fun. The real issue is making sure people have fun and that the game is decently dramatic; features that suck fun and drama out are an issue players will be willing to resolve with you.
Remember combat is not the be-all end-all, it's just one possible thing you can do.
Fate handles all kinds of situations, and physical combat handled via conflicts is just one of them. There are entire Fate scenarios and games where you'll never get into a proper physical fight at all. Then again, nothing wrong with fighting — we've also had Masters of Umdaar games where every second or third session was primarily about physical struggles, and that's fine.
It's pretty common for players coming from a D&D background (such as yourself, I think) to focus the meat of the session on physical combat resolved with bloody violence and murder which, naturally, probably resolves all the worlds' problems if you do it enough. I did it, both the groups I play in did that, a couple of others I have friends in did that. The swashbuckler is going to do well defending in those fairly often.
If your players are in a surroundings like villages where most of the conflicts are social problems and awkward situations to resolve, or they're investigating a mystery and hunting down secrets and scaling towers and snooping on people and stealing things, or they're trying to escape from frankly frightening forces, or they're trying to prove their innocence of a crime they never committed, you're going to have a lot of sessions where no fighting happens at all and the conflict rules might hardly get referenced for a while. There's lots of fun, tension and drama to be had there.
Your swashbuckler's stunt is primarily relevant in fighting. If you have sessions full of fighting, they'll use it a lot. If you have one or two fights every few sessions, they'll shine every now and then but mostly have to also work things out the hard way without making groups of enemies look like pushovers. Try doing those other things as well.