I am currently looking at my favorite Pokemon, Grumpig for the VGC format.

[Revenant L]

I'll won't ever tell someone to never use a certain Pokemon, but to be honest, I would recommend using Cresselia over Grumpig; it outclasses Grumpig in almost every way. However, if you want to use it, then some moves that you'd want to use are Icy Wind, Trick Room, Psychic/Psyshock, Reflect/Light Screen, Grass Knot, Energy Ball, Power Gem, and Signal Beam. If you want to get creative, try Simple Beam, Skill Swap, and Teeter Dance.

True, Sejun Park won with his Pachirisu, but he is widely acknowledged as the best Pokemon player in the world, so if you really want to use Grumpig, you'll have to be very deliberate with your strategy like Sejun is.