I own two Grumpigs, both have the same moveset. One is shiny, but has a Serious nature, which isn't good. I will be going over my non-shiny good Grumpig.
Grumpig @ Rocky Helmet/Leftovers/Focus Sash
Ability: Thick Fat
EVs: 252 Sp. Atk, 50 HP, 150 Spe, 58 Sp. Def
Counters for this Pokemon: Shuckle, Mega Gengar, Chandlure
Those are the main counters, by the way.
You probably won't use this, anyway. Hope you think this is a fun moveset, bye!