Weak toBugGhostDark ×1.6

Attacker	Type	Moveset	DPS
Blacephalon	FireGhost	AstonishShadow Ball	24.7
Pheromosa	BugFighting	Bug BiteBug Buzz	23.8
Darkrai	Dark	SnarlShadow Ball	22.2
Chandelure	GhostFire	HexShadow Ball	22.0
Gengar	GhostPoison	LickShadow BallLegacy	22.0
Absol	Dark	SnarlBrutal SwingLegacy	21.6
Eternatus	PoisonDragon	Poison JabDynamax CannonLegacy	21.1
Dragapult	DragonGhost	AstonishShadow Ball	21.0
Hoopa	PsychicGhost	AstonishShadow Ball	20.6
Lunala	PsychicGhost	Shadow ClawShadow Ball	20.6
Gholdengo	SteelGhost	HexShadow Ball	20.6
Zoroark	Dark	SnarlFoul Play	20.5

Fog boosts the Ghost attacks above. Windy boosts Mega Mewtwo Y.

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

Mega Mewtwo Y's moveset, stats and CP All counters Raid board

Questions people ask

What is Mega Mewtwo Y weak to in Pokémon GO?

Mega Mewtwo Y takes 1.6 times from Bug, Ghost and Dark.

What's the best counter to Mega Mewtwo Y?

Blacephalon with Astonish and Shadow Ball, worth 24.7 damage per second against Mega Mewtwo Y's own defense. The full ranking is above.