Slightly-high CR can be deadly, even on full-resources.
When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party's average level. Such a creature might deal enough damage with a single action to take out adventurers of a lower level....
In addition, some monsters might have features that are difficult or impossible for lower-level characters to overcome. For example, a Rakshasa has challenge rating of 13 and is immune to spells of 6th level and lower. SPellcasters of 12th level or lower have no spells higher than 6th level.... Such an encounter would be significantly tougher for the party than the monster's challenge rating might suggest.
"Challenge Rating", DMG5e p.82, emphasis mine.
Yes, many parties will be able to punch "above their weight" if they're going into something stock-full of spell slots, 'once-per' class features, &c. But I think the advice above is well-heeded in your case. Level 1 characters, in particular, can drop rather fast.
(Just the other week I had a first-level party walk into an ambush. The wizard was first in marching order (!) and a crit from a sling (!!) dropped him during surprise. It only got worse from there. Certainly those players didn't manage it well, but there's no reason your werewolf might not go for the "sparkle-guy" first.)
So how do you gauge the difficulty? Game it out yourself! Grab four prototypical characters and throw them in a room with a werewolf. See how they do. Sure, the action economy's on their side. But the werewolf's going to have resistance to mundane (non-silvered) damage. AngryGM's got anarticle on combat encounters which nicely demonstrates this mini-playtest approach.
[Warning: AngryGM's posts feature vulgar language, though likely no worse than you'll hear on basic cable. Certainly worse than you'll hear here, though.]
But objectives matter...
Is your boss cornered? Is he so single-minded that he'll fight to the death when escape is an option? Why is your boss opposed to the characters? Why are the characters after the boss? These are crucial questions in the encounter's construction. You could throw a CR6 Kuo-toa Archpriest in their way who won't be deadly if...
- He's keenly afraid of any attention being drawn to his presence
- He'll blanch at the sight of any blood
- He's got plans for the party and wants them alive
- He's got plans for the party and just wants to draw them along
- He's in his prepared defenses and just needs to flip a lever to keep the party at bay
- &c. &c. &c.
Your werewolf probably doesn't want to die any more than your players' characters do. Use that.
Shared Storytelling isn't easy
Right now, you and your players are working towards very different things. You want balanced and challenging adventures. You also have a very clear vision of what you want the adventures to entail. They want to do amazing, creative, even zany things.
The problem you are having is the amazing things they would do would break the game balance of the adventures you’ve prepared. But I doubt your players really want there never to be a challenge — they just are champing to do some more of the shared storytelling.
You and your players are telling different stories. The best way out of this is for the stories both sides are telling to weave together more. That might mean things happening in your world that you think are a little goofy. My advice is you do your best to accept player input.
Commit to sharing the story
The good news is you’ve got passionate, creative players that want to take an active role in the game. Yes, that comes with its own challenges, but it can be way more rewarding than players that always just ask, "Is it time to roll initiative yet?"
You should let your players know you want to commit to shared storytelling, being more open to their creative ideas. What you need from them is acceptance that your hours of preparation usually cannot be entirely skipped. Some things will happen largely as the adventure you have prepared, because that is the adventure you prepared. See if they will agree to this deal.
If they really want a more open-ended game style where they can skip content, you could let them know that (like any sane person) you don’t want to throw away hours of work on a regular basis, so you would spend less time on preparation. The encounters would be a lot less fleshed out.
Instead of telling them "No" tell them "Yes, but."
Players tend to be much less upset when you let them try out their idea to limited success, than when you stonewall it. Keep things moving by following Tina Fey's 4 Rules of Improv.
That Bag of Holding trick? The rules say there’s a limited amount of air in the bag. So instead of just saying, "No (ya dummy) that won't work" you might have let them try it, and then use the limited air supply issue to make sure this trick is not overpowering.
In some instances, you can stall them by saying an idea will probably work, but they'll need some downtime to work out the kinks. For example, since the Bag of Holding has a soft bottom, you end up piled on top of each other and it’s just intolerable, and it’s dark, etc. The message is, that’s a fun idea and you can try it later, but right now it just won’t work because it would skip all the preparation.
Have Contingency Plans for Dodged Challenges
Your players like to try to be clever and duck out of challenges you present. Be ready for this, and let winning that race have its own challenges. That MacGuffin they won the race to with their teleport spell might be more heavily guarded than they’d been led to believe. They can still feel clever about beating the Zhentarim there, but now there are ogres. (OK?)
So, you’ll want to identify parts of the adventure that might get skipped, and prepare those a little less. Prepare the “goal” parts of the adventure more.
Use non-combat time to develop shared storytelling
If you let your players come up with clever solutions in downtime, they will feel less stymied, but you will still have time to adjust your adventures to accommodate their clever new tricks.
Taking the specific examples you happened to pick, your players seem to want to skip the overland travel part of their encounters. You might work out a new mode of transport — together with your players — that they think is cool and are happy to use. The key is getting input from everyone. This can happen in-game, where friendly NPC’s find out what the party might want, or out-of-game, where you just talk to the players about what they think might be cool.
Find fun that does not hinge upon your players doing what you want
You mentioned D&D is the lion’s share of the fun you are having right now. And you get disappointed when the players deviate from your intentions. But you should let go of trying to control them, because that will never work.
One way to relax about what the players are doing is to think of your campaign as something that has a life outside of the current game. You’ve basically created an adventure modules — with the amount of work you’ve already put into this, you really might think of putting your materials together as modules that might be distributed, or at least, replayed with a different group. The actions of your current players are just one thing that might happen in the modules.
Give yourself credit
I just wanted to add that, yes, your players sound like a bit of a handful. Seems that folks at the table are very into the game, sometimes a bit too much. Excited debates about what should be possible are part of many campaigns, but hours-long arguments that you don’t enjoy are not something you should have to put up with. (And guildsbounty, for one, offers a lot of good advice about getting them to stop.)
You are in your rights to say, “I’m done arguing about this. Let’s pack up for tonight and I’ll think about it.”
Best Answer
Play the monster like the ambush predator it is
If a monster is slow and has no ranged attacks, then it is most likely an ambush predator that relies on the element of surprise to catch its prey. This seems to be the case for a chuul, which has many abilities well-suited to ambushing adventurers: it can sense magic (including the magic items that adventurers are usually carrying) from far away, it can hide underwater indefinitely, and it automatically grapples with its attacks, preventing its prey from fleeing. So, just by spotting the chuul, your players took away the chuul's most significant advantage. Once the chuul knew it had been spotted, it would have been best for it to run away and hide, trying to set a new ambush somewhere else. As the DM, you can design the map to facilitate this, for example by having the entire floor of the cavern dotted with numerous small pools that all connect to the same body of water under the cavern floor, enabling the chuul to pop out of any pool at any time. That way, even once the players are aware of the chuul's presence, it can still ambush them by popping up where they aren't expecting it. If done well, you'll have your players thoroughly spooked at the unseen predator hiding in the murky depths below their feet.
In any case, the general point is that a slow, melee-only monster is almost certainly an ambush predator, and you should play it accordingly. Think about how this monster gets the jump on its prey, and how it responds when it fails to get the jump on its prey. Even more generally, whenever you choose a monster to throw at your players, think about that monster's abilities and what kind of tactics the monster would employ to make the best use of those abilities. This includes the monster's choice of where to live: for example, an ambush predator will naturally gravitate to an area where ambushing prey is easy. So it's absolutely fine for you, the DM, to design an arena that plays to the monster's strengths. Don't worry if the monster has a low intelligence score, since even a beast with an intelligence of 1 can learn to hunt (or else possess natural hunting instincts). And of course, don't hesitate to search the established lore about that monster for hints.