I worked with the concept for a TF2 campaign called "Machine Attacks" (later renamed 'Retribution') where the act bosses were the TF2 mercenaries with their signature weapons, but beefed up.
I never got to finish the campaign, but I made the bosses and tested them, and they turned out an amazing fight when the party has to change tactics on the fly.
I did have to squeeze my brain trying to balance it in a way that it wouldn't lose the spectacular factor... I wanted the fight to be FUN, first, and tactical a close second... reward the players with the killing of the boss because they figured out the correct tactic, only to then engage the next part with a stronger boss.
originally the bosses were designed by another modder who simply "poof"-ed them back into existence, but I changed the first occurrence of that into a small cutscene that blew the bot up into pieces, and then reconstructed it, T-1000 style.
That, coupled with the dialog and the change in weapon loadout, cemented the concept "These guys have multiple stages".
from there, every boss fight would be an exchange between the boss and their human counterpart who'd despise the existence of each other and what they'd represent: Soldier was stupid, the robot was smart. Heavy was noble, the robot was selfish, etc.
for a multiplayer combat-based game such as TF2, something like a cutscene was unheard of at the time.
when you have such varied gameplay styles as the TF2 classes, it's really hard to not repeat mechanics, but if you lay them out before anything else with the idea of doing that, even if you decree their power without any criteria in a way that they're a stepped progression (regardless of logic), the final product always comes out with a signature "flavor", because you're not always fighting any of the extremes of everything, it's always a mixture.
list all of the weapons, give them a signature style, and number them in order of power, and THEN assign them to the bosses.
the same boss can't wield the strongest and weakest weapon at the same time.... someone else will have to be stronger or weaker at any given point.
Scout was the weakest, so one of his stages was to spawn multiple copies of him. Overwhelms something as Engi's turret or Heavy's minigun, but Spy can simply shock the lot of them at once.
Sniper is vulnerable when he uses his single-shot rifle, so I removed the rifle and gave him a bow.... arrows can be deflected and sent back, so IF you have a Pyro in your team, that's HIS opening... if not, you can still beat the boss by taking cover and shooting at it.
EDIT:
I just remembered I took that as a base to make my RPG database.
the final fight of the tutorial is basically the same skill set of the player characters, built onto another set of characters that are the training masters you face along the way.
it's always the same selection of builds (knight, rogue, wizard and bard), with the same set of skills, but the end result of the fight depends on what you pick to face them.
if you have a knight and the master knight uses aggression on your knight, you no longer have command of that PC knight, you can no longer command it to direct the fight, and the other master characters can target whomever they want.
if the master wizard silences your wizard, you can no longer use it to attack OR heal, if your healer is also down for whatever reason.
if anything, this fight is way more unforgiving than the TF2 fight, because TF2 you always fight many vs one boss... this is party vs party, way more tactical, and way more dramatic... but it's fine, it's intended to be, because the game is dramatic, because there's huge stakes when gods have to intervene to time out the war that's going out of control.
