mainly, just use special grass types or physical faster than him.
here are some good ones that I think could easily take on rotom-W
Venusaur-Mega @ Venusaurite
Ability: Thick Fat
EVs: 202 Def / 252 HP / 54 SAtk
Bold Nature
- Leech Seed
- Sleep Powder
- Giga Drain
- Roar
i mean, mega venu isn't the ultimate rotom counter, but it works perfectly fine. sleep powder to make the chesto berry variant useless and just stall it out with leech-seed or use giga drain.
its pretty much the same for the leftovers variant.
54+ SpA Mega Venusaur Giga Drain vs. 252 HP / 252+ SpD Rotom-W: 152-180 (50 - 59.2%) -- guaranteed 2HKO
not OHKO with giga drain, but venue doesn't fear will's. nor any of rotor's moves, so it should be a good counter
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Breloom @ Life Orb
Ability: Technician
EVs: 252 Spd / 252 Atk / 4 HP
Jolly Nature
- Mach Punch
- Seed Bomb
- Spore
- Swords Dance
breloom is a good answer to rotom outspeeding it and can OHKO with bullet seed or use spore and set up. it may ending up fearing the scarf variant, but these aren't exactly too common and almost no scarf rotom's carries will-o-wisp, so breloom is a very good counter to him
252 Atk Life Orb Technician Breloom Bullet Seed (3 hits) vs. 252 HP / 0 Def Rotom-W: 453-537 (149 - 176.6%) -- guaranteed OHKO
if bullet seed hits two times, the chance of OHKO'ing decreases to 93.8% which is still a pretty high chance
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Excadrill @ Choice Scarf
Ability: Mold Breaker
EVs: 252 Spd / 252 Atk / 4 HP
Jolly Nature
- Earthquake
- Rapid Spin
- Rock Slide
- Iron Head
excadrill is a very useful counter to rotom outspending both the scarfed and unscarfed variants.
just use earthquake combinated with its good ability mold breaker, and it goes like this:
252 Atk Mold Breaker Excadrill Earthquake vs. 252 HP / 0 Def Rotom-W: 318-374 (104.6 - 123%) -- guaranteed OHKO
the physically defensive can be scary thoug….
252 Atk Mold Breaker Excadrill Earthquake vs. 252 HP / 252 Def Rotom-W: 254-302 (83.5 - 99.3%) -- guaranteed 2HKO
but I believe adamant nature fixes that
hope I helped:D