EDIT: I figured out a way and thought i’d share since i noticed i’m not the only person with the problem.
Start at the boss in the back, get 5 stacks. jump to the hurricane boss, kill, jump back. He should give you the 6th stack before he starts channeling. He should finish channeling at this point and will then give you the 7th stack. Once you have the 7th, kill him and quickly jump to the third boss to kill him and get the achiev.
once the bosses reach 90 power they constantly reuse their abilities making it impossible to maintain 7 stacks while having enough time to jump from platform to platform