If I were a WoW boss, I'd have two phases: one in which I am actually multiple characters (5 for 5 man, 10 for 10, etc) with separate health pools, each standing for a role in the raid. I would then function like a raid group would (attack the healer, interrupts, tank the DPS, heal party members, use class cooldowns and moves, etc.) and force players to obey an agro table of their own.
Phase 2, after the first group was killed, I/we would then come back with identical names, appearances, and classes to the opposing group and the raiding group would be afflicted with a permanent (for the fight) Maddening Shout (sans the damage, I just want to shake them up with it). To shake things up, my clones would also copy anything the original said.
I would have the most transmog-worthy gear of any boss and would have an Old Gods themed lair, to account for the madness.