#0139 · Gen 1

Omastar — Best counters

Weaknesses

grass×2.56 double weaknessfighting×1.60ground×1.60electric×1.60

	Pokémon	Fast move	Charged move	Weather	Score
1	Sceptile (Mega)	grassBullet Seed	grassFrenzy PlantELITE	Clear	10019.4
2	Kartana	grassRazor Leaf	grassLeaf Blade	Clear	9217.9
3	Shaymin (Sky)	grassMagical Leaf	grassGrass Knot	Clear	8316.2
4	Venusaur (Mega)	grassVine Whip	grassFrenzy PlantELITE	Clear	8215.8
5	Groudon (Primal)	groundMud Shot	grassSolar Beam	Clear	8015.5
6	Roserade	grassMagical Leaf	grassLeaf Storm	Clear	7815.1
7	Zarude	grassVine Whip	grassPower Whip	Clear	7714.9
8	Xurkitree	electricThunder Shock	grassPower Whip	Clear · partial	7714.8
9	Meowscarada	grassLeafage	grassFrenzy PlantELITE	Clear	7314.2
10	Decidueye	grassMagical Leaf	grassFrenzy PlantELITE	Clear	7213.9
11	Lucario (Mega)	fightingForce PalmELITE	fightingAura Sphere	Overcast	7213.9
12	Tapu Bulu	grassBullet Seed	grassGrass Knot	Clear	7213.9
13	Tsareena	grassMagical Leaf	grassGrass Knot	Clear	7113.7
14	Rillaboom	grassRazor Leaf	grassGrass Knot	Clear	7013.6
15	Sceptile	grassBullet Seed	grassFrenzy PlantELITE	Clear	7013.5
16	Breloom	grassBullet Seed	grassGrass KnotELITE	Clear	6913.4
17	Gallade (Mega)	fightingLow Kick	grassLeaf Blade	Clear · partial	6913.4
18	Lilligant	grassMagical Leaf	grassSolar Beam	Clear	6813.2
19	Chesnaught	grassVine Whip	grassFrenzy PlantELITE	Clear	6813.2
20	Regieleki	normalLock On	electricThunder CageELITE	Rainy · partial	6813.1

Grass hits for ×2.56 — Omastar's Rock/Water typing is double-weak, so those attackers outperform their score gap.
Watch its Ground/Water/Rock moves — counters like Shaymin (Sky) and Groudon (Primal) take super-effective damage from them.
Clear weather boosts Grass moves ×1.2 — the best time to battle Omastar.

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