#0351 · Gen 3

Castform (Sunny) — Best counters

Weaknesses

	Pokémon	Fast move	Charged move	Weather	Score
1	Kyogre (Primal)	waterWaterfall	waterOrigin PulseELITE	Rainy	10012.9
2	Groudon (Primal)	groundMud Shot	groundPrecipice BladesELITE	Clear	9812.7
3	Diancie (Mega)	rockRock Throw	rockRock Slide	Partly Cloudy	8911.5
4	Garchomp (Mega)	groundMud Shot	groundEarthquake	Clear	8711.3
5	Landorus (Therian)	groundMud Shot	groundSandsear StormELITE	Clear	8511.0
6	Swampert (Mega)	waterWater Gun	waterHydro CannonELITE	Rainy	8410.9
7	Tyranitar (Mega)	rockSmack DownELITE	rockStone Edge	Partly Cloudy	7910.2
8	Blastoise (Mega)	waterWater Gun	waterHydro CannonELITE	Rainy	7810.1
9	Gyarados (Mega)	waterWaterfall	waterHydro Pump	Rainy	7710.0
10	Sharpedo (Mega)	waterWaterfall	waterHydro Pump	Rainy	769.9
11	Kyogre	waterWaterfall	waterOrigin PulseELITE	Rainy	769.9
12	Aerodactyl (Mega)	rockRock Throw	rockRock Slide	Partly Cloudy	769.8
13	Groudon	groundMud Shot	groundPrecipice BladesELITE	Clear	759.7
14	Rampardos	rockSmack Down	rockRock Slide	Partly Cloudy	759.7
15	Excadrill	groundMud Slap	groundEarthquake	Clear	719.2
16	Eternatus	poisonPoison Jab	dragonDynamax CannonELITE	Windy · partial	719.2
17	Quaquaval	waterWater Gun	waterHydro CannonELITE	Rainy	709.1
18	Rhyperior	groundMud Slap	rockRock WreckerELITE	Partly Cloudy · partial	709.1
19	Mamoswine	groundMud Slap	groundHigh Horsepower	Clear	709.0
20	Greninja	waterWater Shuriken	waterHydro CannonELITE	Rainy	699.0

Castform (Sunny) is weak to Ground (×1.6), Rock (×1.6), Water (×1.6).
Watch its Fire/Normal/Grass moves — counters like Kyogre (Primal) and Diancie (Mega) take super-effective damage from them.
Rainy weather boosts Water moves ×1.2 — the best time to battle Castform (Sunny).

Rankings are estimated cycle DPS (attack stat × move damage ÷ cycle time, with STAB and type effectiveness). Not a full battle simulation.