Weak toGroundBugGhostSteelWaterGrassDark ×1.6

Attacker	Type	Moveset	DPS
Kartana	GrassSteel	Fury CutterLeaf Blade	28.4
Blacephalon	FireGhost	AstonishShadow Ball	28.0
Pheromosa	BugFighting	Bug BiteBug Buzz	27.0
Metagross	SteelPsychic	Fury CutterMeteor MashLegacy	26.7
Inteleon	Water	Water GunHydro CannonLegacy	25.3
Kyogre	Water	WaterfallOrigin PulseLegacy	25.2
Darkrai	Dark	SnarlShadow Ball	25.2
Chandelure	GhostFire	HexShadow Ball	25.0
Tinkaton	FairySteel	Fairy WindGigaton HammerLegacy	24.9
Gengar	GhostPoison	LickShadow BallLegacy	24.9
Groudon	Ground	Mud ShotPrecipice BladesLegacy	24.7
Absol	Dark	SnarlBrutal SwingLegacy	24.4

Rainy boosts the Bug attacks above. Partly cloudy and Windy boost Iron Boulder.

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

Iron Boulder's moveset, CP and shiny All counters Raid board

Questions people ask

What is Iron Boulder weak to in Pokémon GO?

Iron Boulder takes 1.6 times from Ground, Bug, Ghost, Steel, Water, Grass and Dark.

What's the best counter to Iron Boulder?

Kartana with Fury Cutter and Leaf Blade, worth 28.4 damage per second against Iron Boulder's own defense. The full ranking is above.