Yeah, the Big Bruiser is going to get hit a lot. That's what he's there for!
But a 12 Toughness is HUGE. He also has a much higher wound threshold than other characters--he starts making Up Checks at 50 Wound Points instead of 35. This is also huge! You'll have chances to describe how badass the Big Bruiser is by saying "The snake person winds up and swings its enormous tail at you, and lands a blow which would send any normal person flying across the room--and you just stand there, and barely feel it!"
In any Feng Shui 2 game, but especially in a one-shot or a standalone game, the heroes should be prepared to sacrifice themselves against a Boss. That's the nature of the heroic bloodshed style embodied by FS2. In my own campaign, we treated it less like a John Woo movie and more like a season of Arrow or The Flash, where the threat of death was much much lower. But in a one-shot, all bets are off!
Even if all the heroes die, you can give them a last chance to trigger a bomb or destroy a magical urn or get off one last shotgun blast which could kill the enemy. Maybe you even ignore the stats at this point, and just describe it happening as the last hero's life slips away from them, they know that at least they have rescued the captives from the sorcerer's lair, or destroyed the plans for the bomb which was going to destroy Hong Kong...
In setting up your final battle, I'd encourage you to be creative about the environment. The Big Bruiser is Strong as an Ox, so he can pick up things most characters can't. If there's some scaffolding or big sculptures, or a giant block of supercomputer servers, or a tree... these are all things the Big Bruiser could throw or swing at a villain. Be creative with letting environmental effects mess with the Boss character. If the Big Bruiser topples a giant Buddha sculpture onto a Boss eunuch sorcerer, there's nothing wrong with saying he takes some damage AND gets knocked down 3 shots on the Shot Counter as he uses his magical blast to explode the sculpture from underneath.
Use your descriptions to give the bruiser ideas if it doesn't occur to them. "The eunuch sorcerer cackles as he ducks behind an enormous stone Buddha which is 20 feet tall. The statue rocks a bit on its base as the Full Metal Nutball unloads his shotgun into the snake people surrounding him. The sorcerer calls out, 'Don't you understand, even your great strength can have no effect on the magic at my command!!!'"
Any Big Bruiser worth their salt should get the idea and try to push that statue over onto the sorcerer.
For the Big Bruiser, keep in mind Mounting Fury I. Every time the bruiser misses, they increase their odds of hitting on their next attack. They can also act as a kind of crowd control. Your Boss may be really powerful, but they're not going to ignore a huge dude swinging a parking meter or a telephone pole at their head! The bruiser's first 2 or 3 shots may even miss the Boss, but eventually those bonuses will add up and they'll land a hell of a blow!
If you wanted to highlight the Big Bruiser further, you could trigger a poison gas trap or something which would require a Constitution Check. The bruiser excels at those.
Another thing I often do for the boss is have some very specific things which the heroes need to do, above and beyond just "kill the boss". Maybe he's burning incense in braziers around the room and his cultists are protecting those burning fires, because they're part of a spell which gives the boss power.
Maybe the boss has the plans for the bomb on his computer and your real objective is to get to the computer and steal or destroy the plans. You'll need to get past the boss to do it, but the objective isn't necessarily just "Kill him". Maybe there are captives to rescue or a cloning machine to destroy.
Any of these things could be behind a huge door which would be impassable for almost anyone--except for the Big Bruiser, who can not only stand there and spend his shots making Strength checks to lift up the huge door as everyone passes through it, but can also soak up the damage from the mooks and foes trying to take him out as he does it. A shotgun blast doing 14 points of damage? 2 points gets through. A handgun doing 11 points? He shakes it off!
This was a bunch of stream of consciousness rambling, but I hope it helps
The main thing I'd recommend for a one-shot, especially introducing new players outside of the context of a big campaign, is... don't worry about the rules. If something is cool, just do it. If a player has a great idea, just go with it. Feng Shui 2's a great system to give the players the incredible experience of being way-over-the-top action movie heroes!