The most reliable counters are probably Blissey, Chansey, and Heatran.
Blissey and Chansey are basically bulky enough to take any hit from it, since most of them will be special. And they can stall it out with the Toxic + Wish + Protect combo.
Heatran just resists Solar Beam, Air Slash, and Dragon Pulse and gets a Flash Fire boost from Fire Blast, so it makes a great counter. But you gotta watch out got E-Quake variants, because I think those will be fairly common.
As for just flat out KO'ing it, it's not that hard. Anything that can outspeed and hit it hard does fine. Priority from Talonflame and Azumarill does nicely too.