Weak toBugGhostDark ×1.6

Attacker	Type	Moveset	DPS
Blacephalon	FireGhost	AstonishShadow Ball	28.9
Pheromosa	BugFighting	Bug BiteBug Buzz	27.9
Darkrai	Dark	SnarlShadow Ball	26.0
Chandelure	GhostFire	HexShadow Ball	25.8
Gengar	GhostPoison	LickShadow BallLegacy	25.8
Absol	Dark	SnarlBrutal SwingLegacy	25.3
Eternatus	PoisonDragon	Poison JabDynamax CannonLegacy	24.7
Dragapult	DragonGhost	AstonishShadow Ball	24.6
Hoopa	PsychicGhost	AstonishShadow Ball	24.2
Lunala	PsychicGhost	Shadow ClawShadow Ball	24.2
Gholdengo	SteelGhost	HexShadow Ball	24.1
Zoroark	Dark	SnarlFoul Play	24.1

Fog boosts the Ghost attacks above. Windy boosts Grumpig.

Level 40, 15 attack IV, costed against Grumpig's own defense and typing, one per evolution family.

Grumpig's moveset, CP and shiny All counters Raid board

Questions people ask

What is Grumpig weak to in Pokémon GO?

Grumpig takes 1.6 times from Bug, Ghost and Dark.

What's the best counter to Grumpig?

Blacephalon with Astonish and Shadow Ball, worth 28.9 damage per second against Grumpig's own defense. The full ranking is above.