Weak toSteel ×2.56GroundWaterGrass ×1.6

Attacker	Type	Moveset	DPS
Metagross	SteelPsychic	Bullet PunchMeteor MashLegacy	34.5
Tinkaton	FairySteel	Fairy WindGigaton HammerLegacy	31.9
Lucario	FightingSteel	Bullet PunchMeteor Mash	31.8
Dialga	SteelDragon	Metal ClawIron Head	30.5
Genesect	BugSteel	Metal ClawMagnet Bomb	29.8
Archaludon	SteelDragon	Metal ClawFlash Cannon	28.7
Excadrill	GroundSteel	Metal ClawIron Head	28.4
Zacian	FairySteel	Metal ClawIron Head	28.3
Zamazenta	FightingSteel	Metal ClawIron Head	28.3
Kingambit	DarkSteel	Metal ClawIron Head	26.6
Melmetal	Steel	Thunder ShockDouble Iron BashLegacy	25.9
Empoleon	WaterSteel	Steel WingFlash Cannon	25.3

Snow boosts the Steel attacks above. Overcast and Partly cloudy boost Mega Diancie.

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

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

Questions people ask

What is Mega Diancie weak to in Pokémon GO?

Mega Diancie takes 2.56 times damage from Steel, and 1.6 times from Ground, Water and Grass.

What's the best counter to Mega Diancie?

Metagross with Bullet Punch and Meteor Mash, worth 34.5 damage per second against Mega Diancie's own defense. The full ranking is above.