I'm talking kill Dialga with Gengar Prime/X and Cursed Droplets/Compound Pain. But if you CAN get DGX in there hand that's even better. Or if you Looker's them and hope they draw it. But that'd be VERY luck based.
Hunter, they will most likely pick up their Garchomp C X or something good like that. (Depending on the game state, that is) Then you can GET RID of their heal. If you can get rid of their Dialga G basics, that's not bad either.
Edit: Mistakes, and added just a good addition to Gengar that may/not have been forgotten.