I'm assuming you want six team members, if that's too many just choose from these: Walrein, Granbull, Scizor, Hitmontop, Sceptile and Dusclops.
Remembering something, I went North from Mahogany town and got Sludge Bomb. In the following gym, Peter one-shotted Pryce's Seel with it, Dewgong put...