A little boss winning info:
Badspot: Wins if you play him right, which means whipping out a wand and slamming the guys who try to backstab you when they least expect it, as well as being good aim with shockwave. Loses if you don't watch your back. Recommended class: Spy (if it's a less experienced badspot) or Pyro (if he's playing it right)
Ephialtes: Wins if he hangs around up top and shocks people into walls etc. but loses if he stays on the ground too much. Once I used Guns Akimbo plus Firebug(pyro) and slowly chipped over 75% HP off of an Ephialtes. Recommended class: Pyro, and buy yourself some akimbos. If he throws you up high, you can land it with pyro, and akimbos deal a lot of damage.
Mocheeze: Wins if he abuses his missiles and never stays on the ground and constantly moves in ways hard to predict AND doesn't spawn on the shelf (if he spawns on the shelf someone tends to tank him from underneath and kill him in an instant). Loses if he holds still, if there are too many heavys spamming bullets, or if he doesnt take out the snipers soon enough. Once a Mocheeze managed to make ONE JUMP and I sniped him in midair WHILE HE WAS MOVING. I laughed so hard at how easy it had been. Recommended class: Sniper (if you're good with scope mode) or heavy (if you prefer the machine-gun approach)
Kaje: Wins if he spawns fast and puppy-guards the turret but still jumps around enough that the snipers have trouble killing him (the OSR pierces vehicles and so can even shoot him through the back of the roosterpit if it hits him), loses if he doesn't watch the turret or if he holds still enough for snipers to get him. Also loses if a REAL IDIOT jumps out of the mech. Once a Kaje jumped out of the mech, a normal jumped in and shot him dead with it. Recommended class: Pyro (if you think you can reach the turret in time), Sniper (get him through the mech), or Engineer (sentry rockets can bypass his insane defense)
Penthilesa (or however you spell that): Wins if he can aim spears at moving targets and keep moving as well. Loses if he doesn't watch his back. Recommended class: As Badspot, backstab the idiots or torch the experienced.
Celestia: Wins if she constantly jumps and always keeps the flame on. Loses almost never, though every now and then one won't jump enough and will get Akimbo'd to death. Recommended class: Akimbos+Pyro or Cloak and Dagger (so you can camp on the cap point and when everyone else is dead jump on it's back and stab it to death). Also note that Celestia only ever loses when Normals manage to stall out of time. RECOMMENDED CHANGES: Reduce that health some.
Samus: Don't get me started. She almost never loses, even with the lowered health. A truly frightening expert abuses invisibility to sneak around appear, rocket someone, and disappear again. Loses when she fails to dodge the akimbos. Recommended class: Pyro, and buy some Akimbos. Nothing else seems to work on this boss. RECOMMENDED CHANGES: Remove the invisibility or get rid of the rockets, in addition to the nerfed health.
Rotondo: This is the boss I pick. Wins when he gets rid of the snipers, pyros, heavys, and akimbo-users fast, loses when the slight delay between click and clawing screws up all his attacks (speaking from experience here, it helps if Rotondo turns his light on so he can figure delays more accurately). Recommended class: Pyros with akimbos tend to last the longest against Rotondo, unless it's a very nooby boss who gets backstabbed fast. Pyros work well because they can look slow and then jump away just as Rotondo approaches. Once I killed a rotondo by akimbo-spamming and jumping over his head whenever he got close. He almost never loses, but I'm not sure how to balance his abilities.
This is King of the Bill, BL_ID 30051, speaking from experience on Pecon's server.