the thing is though that tf2 classes are not necessarily the best choice for boss battles
i mean, yes, they have their place (in team fortress, for instance) but in an environment where you are literally free to do whatever you want to optimize the game and make it more fun instead you're just stripping content from somewhere else and copypasting it into your server instead of actually putting thought into them and what they would imply for the rest of the game
for instance: heavies and medics are the most stupidly applied classes ever in boss battles
the heavy- while he's 'designed' to be lastable against the boss (but not for any reason apparently, just to be different from the rest of the classes)- is too slow to avoid attacks and still killed by half of the instant-kill moves that the bosses have. but, there's no saving the guy as a playertype because, as a concept within a boss fight, a player that is too slow and clunky to out-maneuver (and is instead just a walking tank) a boss whose schtick is basically that he is a walking tank that can instantly kill you unless you approach him the right way* is just a liability. you can't make him tougher without making him a pain for the boss (not in a 'oh, man, this guy is dangerous and i should kill him first' kind of way but in a 'wow it's not fun at all to shoot this guy for 30 minutes while the rest of his team shoots at me' kind of way) and you can't make him dish out more damage or something because he'll still be instant killed by the boss. combining these two examples (there are many more fallacies that i don't want to address because i think i've made my point) will just make them overpowered entirely, and not only make it no fun for the boss to kill him but it'll cause everyone to play heavy because he's so overpowered, it'll cause the bosses nothing but stress because the other team is now stronger then them and the only real effect is that now the server is completely turned on it's head
the optimal solution to this is coming up with a new system entirely (classes are fine, but team fortress classes where team fortress classes don't belong aren't) that all have separate, useful strengths that all have a distinct place within the arena
for instance there could be four classes: one with a raycasting assault rifle and a small rocket launching pistol designed to do damage to the boss and play as a sort of 'not-extremely-special' character that, for gameplay balance and for the sake of being interesting, can do something neat like be the only one who can use 'field items' (which could be dropped from the boss, serving as power-ups), another that heals everyone it gets close to (healing specific targets more effectively with a special 'syringe') armed with a shotgun designed to help keep everyone alive and serve as a rally point, a third with a powerful, precise railgun and SMG that moves quickly but has less health then the 'assault' class who can do extreme critical damage by hitting weak points but isn't as useful without a team to draw fire and a final class that can dish out lots of damage with an enormous chaingun (like the heavy, but not the heavy) that runs quickly enough to dodge dangerous weapons and has a damage 'tolerance' that allows him a little bit of room for error before he starts getting hurt from the boss (for instance, the first few 'little' bullets that hit him are simply soaked up, and then anything that hits him afterward begins to do damage: bigger projectiles like rockets doing full damage on the first shot, but that's because they have a higher threat level and thus should be instantly dodged)
*pecon's boss battles do not have anything as intricate and interesting as a concept like this. i mean, the bosses could have something like weak points or patterned attacks or explicit maneuvers they have to pull off or telegraph their attacks and force players to react in time or anything at all but it's really just a glorified AiO.
a really loving unbalanced AiO that's downright agonizing for anyone who isn't the boss, and needs to be repaired