Never Grass Knot Rotom-W. GK's damage is based on weight, and Rotom-W is an insanely light Pokemon. If you want to take it down, Mold Breaker Excadrill is your best bet. You can tack on Assault Vest for a lot more special bulk if you invest 252 into SpD, it will be able to tank Hydro Pumps easily.
4 SpA Rotom-W Hydro Pump vs. 192 HP / 252 SpD Assault Vest Excadrill: 170-204 (41.5 - 49.8%) -- guaranteed 3HKO
Also, Gastrodon and Stockpile Quagsire are textbook counters to Rotom-W. Gastrodon is immune to both of Rotom's attacks, and can set up freely. Mega Heracross and Breloom secure OHKOs easily with Bullet Seed, and setting up DD and EQing with Mega Gyarados will take care of it in a pinch. Gliscor can stall Hydro Pump with its standard SubProtect rotation until it misses, Toxic it, and continue the stall until it dies. Toxicroak can set up a Substitute on the turn that Rotom should Will-o-Wisp, Swords Dance, and KO it with Drain Punch with a healthy Sub for another turn. Mega Charizard Y sets up Sun, lowering the damage of its Hydro Pump, and can KO back with Sun-powered Solarbeam. Mega Charizard X can M.Evolve, tank the HPump or Volt Switch, DD up, and sweep. Guts Conkeldurr (esp. with Assault Vest) doesn't give a damn about anything Rotom can do to it, since it tanks all attacks with AV and gets power boost off Will-o-Wisp.
His moves are also REALLY predictable (if you have a Water type, he'll VSwitch - swap to a Ground type. If you have a Ground/Rock/Fire, swap to a Water resist or something with Water Absorb. If you have a physical attacker, he'll Will-O-Wisp - swap to a Fire type. Keep doing this until you get a SAFE switch into a desirable matchup, like your Exca vs. the Rotom).
Rotom-W is a little **** that has had some huge impacts on the meta (for example, the main reason Gastrodon was brought to the limelight of OU was the fact that it hard-countered Rotom to every extent).
Or, just swap in any Grass type faster than it.