Aight, so unfortunately, it's been a long time since i've played Rejuv, so please consider that my opinions might not be accurate here.
The 3 mons that saved me in my playthrough were Roserade, Crobat and Magnezone.
Roserade, because she was consistant throught the game, bring some valuable assets with good coverage.
Magnezone, mostly because of sturdy, while he helped paralyzed the foes.
Crobat was usually the one that came in clutch for me. Because he camed with brave bird, he extremely well against a lot of major battles (Namely flora and, suprisingly, both souta and rift gardvoir).
I could have also talked about Alolan Muk, but i'm good with those three.