Have a pokemon that knows surf and a pokemon that knows muddy water(which is a marshtomp or a swampert). Attack with surf and muddy water until its all out and use a pokemon that has recover. Use an attack that does massive damage (like any punch) to 1 pokemon. When u kill at 1 of the pokemon use recover(if you dont have much HP)and use the same move you did to knock out the other pokemon to whipe out hte trainer by attacking the pokemon left.