Diggersby is not as bad a Pokemon as people think it is. It has HUGE fkin POWER!
Use Prankster to your advantage, maybe put a Leech Seed, TWave, Curse. Then stall.
On the other hand you could try to outwrestle it which is what I did. I Paralyzed it then had Noivern Super Fang twice and work on paraflinching it.
Then Swampert came in and tanked 2 hits and managed to KO Arceus even after Recover.