Ah, so I see you took my failed attempt at a counter that pokemon thread and reworked it for OU. I have to say the "target a day" system works a lot better on this highly populated forum than the "counter the last poster" system I used.
Also I find it amusing that you posted trick-scarf Starmie for this week, since it's a set I suggested in the last OU CCAT, though I believe we came up with the set independantly of one another.
Anyway, onto business.
Rotom-W @ Leftovers
Nature: Modest
EVs: 112 HP / 252 SpA / 144 Spe
- Volt-Switch
- Hydro Pump
- Hidden Power Fire / Will-O-Wisp
- Pain Split
The worst Starmie can do to Rotom-W is hit it with Thunder, which will never 2HKO with Stealth rock. Trick, as annoying as it is, doesn't harm Rotom-W as much as it would most counters since Rotom-W runs scarf half the time anyway. Volt-Switch, on the other hand, puts Starmie at 5% health at most and OHKO's 2/3s of the time without stealth rock, leaving it open to being finished off by so much as a tap should it hang on. You could run thunderbolt for a 100% KO of course, but usually the other moves have more utility. Pain split can heal off damage incurred from Thunder should you need to. Really, the only real threat is paralysis from thunder, but that requires an awful lot of nasty hax and usually Starmie won't be dumb enough to stay in anyway. And when it does switch out, you can volt-switch away to a counter, getting some serious momentum. Well unless you get tricked and then locked into volt-switch and the opponent has an immunity I suppose. Still, a better answer than most. You could also run a different spread to better take hits from more special attackers with more special defense or something, but I'm not familiar with that spread so I just posted the old one.