First, the operation as a whole should have them as individual bosses with unique mechancs for each one.
As final boss - all of them at the same time in a 3-4 phase fight.
Phase1 - counsil style battle. More like 1v1 duel with tank swapping or player swaping because of some debuffs (analogy to "leech" "doom" or something toward those lines).
Phase2 - "fear" phase. Dread masters will use their fear ability, but it needs all of them to function properly. Mechanics would involve separation of them or focusing specific member at spesific moment. Or it can include "nightmares" - representations of bosses we fought before ( in EV / KP / Denova / Asation).
Phase3 - inseperable entity phase. After phase2, when all of them are "dead", their combined strenght/power/essence will be the next challenge.
Phase4 - Crumbled reality. The whole place (station / room / planet) begins to fall apart, with ground shattering / floating around the place. That would be the dps race within the "no physic laws" enviroment.
This would be a VERY weird operation. I can imagine the first 3 bosses being 2 dread masters each, then some kind of puzzle, then the finale being all 6. It sounds like a bucket of fun.