Well, as you stated, a smart approach when this happens is to just ask the player, but with that out of the question here are some options I might use in this situation:
Give objectives.
Try to add in something extra to the fight, maybe a lever to pulled, or even better, a door to be unlocked (either locky-picky, or with hacking). A cool way to use this is to have a battle that the group (including him) isn't ready to face, but giving them an "out". Just make sure that they know that they can't win this fight — nothing is worst than hubris getting in the way of an awesome escape scene. With this the rest of the group still has stuff to do (the hacker can try and open the door while someone strong might try to barricade them a bit, etc.) while he is getting challenged to "real" fight.
Boss with minions. This is less fun in the long run, but here and there it could do you wonders — have a fight that centers around a main baddy. This leads to one of two options:
a) That he has to focus all of his efforts on the baddy, giving them time to fight the goons; or the somewhat better b) of letting them take on the baddy (merely being strong isn't enough, it needs some skill or combination of skills to kill him) letting them show off what they are good at, and giving him the feeling of power that comes with mowing down 7 baddies in 2 rounds.
Split the party either before the fight, or through the sheer size of it (he can't be everywhere at once, can he?), so that he might take out everyone in one room, but that doesn't quite cover the next room where the other part of the group is fighting.
And yes, having less combat, even in Savage Worlds, is a good thing — it helps you evolve as a GM, and it's often more fun than "another random fight". Fights are cool, but they aren't the only cool thing, not in role-playing games: the coolest thing, is playing a role.
At a convention, playing time is a rare commodity and I generelly recommend not being "that" player who holds everything up with discussions. For the record: I'm from Germany, so my convention experience might differ greatly from yours, all of the following is just my subjective opinion on the matter.
So if what the GM is doing is ruining your fun at a convention game, yet isn't interested in changing anything about that, the main question in my mind is: Does it affect the other players? If they feel the same as you do, I feel it's valid to point out that the way he runs the game isn't fun for the players.
But if they don't seem to mind either, then the path of least resistance is to just get up and leave, be friendly, be polite (the guy wasn't trying to ruin your fun on purpose, after all), try to avoid in-depth discussions on the spot (those tend to get heated and personal quickly and hold up the game for everyone) and just say it's not working out for you. If the GM is honestly interested, offer to talk to him afterwards and calmly explain the matter then.
I generally would only inform convention staff if you feel like the GM was actually trying to ruin the players fun on purpose, I know it can suck if you don't get the game you like, but in my experience, there's a big continuum of styles from "complete freeform" to "everything 100% RAW, all the time, no exceptions", and I don't think it's worth making too big a fuzz about a GM running the game in a different way. Just make it a point to avoid that GM in the future.
Best Answer
Since there's no specific mention of the number of players, lets look at the challenges in the pdf. We'll go over each plot point, and see what dice/rolls are needed to succeed. There's a quick tl;dr at the end if you just want my opinion on party size. Potential spoilers below!
It will prove advantageous for most players to be drunk (unless they rely on smarts!)
A d8 in Common Knowledge gives slightly better than 50% chance of success. However, a raise on this roll informs the PC that a legion of undead lie below. At this point, my PCs would probably attempt to set the entire catacombs on fire; ymmv. Regardless, they should be able to grab some quick supplies from above. I would probably have the NPC soldier join them upon learning.
A piece of rope, a ladder, even a broken door can circumvent this challenge if no-one has the strength for it.
Assuming your party has torches, this is pretty trivial. Otherwise, stomping them works well.
How the group uses the surprise round will largely determine how difficult this is. Setting the zombies on fire will mean your group wins unharmed; charging in blindly might result in a few wounds. Since the backstory explains that the PCs have already encountered zombies, they should be familiar with the headshot weakness.
Good thing you drank that alcohol, right? A d6 vigor gives a 50% chance of passing this drunkenly, and I would house-rule that a PC can only be poisoned by the gas once.
The first real challenge. A notice of 4 spots the golem's weakpoint, and a raise prevents the golem's surprise round. Once the weak point is spotted, a called shot at -4 will seriously wound the golem. A shooting skill of d10 gives a 50% chance of hitting the call shot. The GM can provide additional zombies (if any) based on the parties success this far.
The necromancer's main strength is his bolt and zombie powers. If you've prevented his ritual, he's a slightly buffed novice spell caster. If he completes the ritual, then he has 50 PP and gets more if he kills a PC. He's easiest to kill with arrows or a greatsword to the head, as his advantage is in firing long-range bolts.
So how does all this help? For one, we can see that most of the challenges require abilities easily obtained by novice characters. A d10 shooting skill is the highest required die for a 50% success chance, and passing that roll will make short work of the bone golem.
The combination of skills required isn't really varied either. One character focused on smarts (knowledge, notice, spellcasting), one character focused on agility (parry, shooting), and one character focused on vigor (tanking, melee fighting) should be able to pass all the challenges. A specialized gun-toting holy warrior could likely solo the entire challenge.
My advice, based on these stats and a little gut intuition, is to have a minimum of 3 party members and a max of 5. If the PCs make it to the final chamber with full resources, add some zombies to increase the challenge.
A lot of the difficulty will also depend on the experience of your players. Experienced Savage World players know the best way to position themselves on the board, take advantage of movement, and minimize area of effect damage against them. Newer players who are still learning these concepts will struggle more.
tl;dr: For this specific challenge, a party size of 3-5 seems ideal. Less characters will have trouble with some of the challenges, more will make them trivially easy.