Rather than the standard boss model, why not have a completely stationary boss, sort of like an incredibly deadly trap. The room would be specifically designed with 3 levels players can jump up to, and the boss would have three sections to it's body.

Floor Section
This would be the most deadly section, damageable by melee or ranged. A constant pillar of fire would shoot out from boss to wall, circling the entire room slowly, and every time it would make 1 full rotation, the entire floor would explode, killing anyone who didn't jump up to a higher level.

Mid Section
This would be a ranged only section, where melee could hit, but they would have to be jumping up constantly to maintain damage, as there are no platforms close enough to stand on and dps. Ranged dps would be able to dps at will, but in conjuncture with the bottom level, whenever a player on the bottom was hit by the fire, or the explosion would occur, all players on the middle level would get sucked into the totem. You'd have a few seconds to distance yourself before a secondary explosion would occur, only in melee range.

Top Section
This would be the melee section. Ranged can dps here, but there are no long range platforms. Similar to the mid section, this part of the boss would also have trigger mechanics from the bottom. Instead of pulling people in, if anyone on the bottom level is hit by fire, and when the explosion occurs, all melee range people on the top will be blown backwards. Since this blowback occurs at the time of the bottom level explosion, melee players will be knocked down to the second level and pulled in with the ranged, and needing to avoid the second melee explosion.

This is just a basic idea, and obviously it can have other mechanics, such as adds, but hopefully this can spur some creativity and get new ideas to go along with it.