When I first played, I had a simple team. Empoleon, Staraptor, Roserade, Lucario, Raichu, and Rapidash were able to get me through it all, but really I find Empoleon was the only thing needed.
After that I began experimenting with new teams. I've had my Blastoise go at it alone and win easily (I coddle that thing too much), and overall I find the Sinnoh Elite 4 to be the easiest, hands down. ANY Water type with Ice Beam/ Blizzard will do just fine. :]