Simply put - three waves, with each wave being one of the four bosses (FK/Spider/Dragon/IG). Every week, the specific three as well as their ordering rotate randomly. Exact same structure as current dungeons - standard 1-25, hardmode 1-10.

The only change I'd make would perhaps be to weaken FK, since I'm not sure if it'd really be possible at all to do FK10 with the same team as any of the other three. But I'd really only make the change for FK6-10 - just lower his speed for those four levels. Everything else can stay the same.

The reward for Boss Rush would be a reward from each boss. The energy cost would be marginally higher - rather than 20, it'd be 30. Perfect for endgame players, and extra perfect for dealing with the stupid Dungeon Divers events.