Putting the format aside, what about some of these?
1.) Ride 'em, Cowboy: Players must climb atop or bridle a huge solo enemy (or perhaps shave off a piece of its eye horns). The creature is too rare, valuable, or immortal to kill, so the party has to use distraction, non-lethal damage, or...something else, to get a party member up top. (I did this in a 3e campaign with a rampaging Apparatus of Kwalish).
2.) Guessing Game: The monsters look like you and your friends, as if you were mixed up in their Mirror Image spell. On any given turn, it's hard to tell who is your ally and who is the monster. As a minor action, the foes can mix themselves up again, meaning that whatever clues you used to tell them from your allies has to be done again.
3.) Fading Fast: Each round, some of the party's power is lost, perhaps as part of some draining spell. Each round the PCs age, lose levels, or lose memories. The enemy must be defeated quickly, but every round it gets harder.
4.) There can be only One: Each round, any player that fails to meet a certain condition is instantly removed from play (Ex. Whoever caused the least damage, failed to hit their target, didn't fight with honor, forgot to say "please")
Eh. I'm just throwin' stuff out there.