With Serra you break the field and it's practically GG. On my own Ground type mono I've gotten to Samson (and got bored XD) with Torterra, Camerupt, Whiscash, Mamoswine, Nidoking and Flygon. I also used Dugtrio and Onix for a period of time, and Dugtrio's speed was great.
Also, RUN SPECIAL ATTACKER NIDOKING. Sludge wave is available relatively early, it learns Earth Power through level up, Flamethrower later on and Megahorn for coverage if you run into Psychic types with good SpDef