It would be hard to beat him without evolving it in to swampert first, because then swampert can learn muddy water. But marshtomp's most effective move should be Mud shot.
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You fight him after you get 4 gym badges
go to mom she will give you the amulet coin
I would say any fighting types.
After you get the Balance Badge (Norman) talk to Wally's dad and he will give you surf
all lv.100s because it smacks the hardest way is to all the same lv.s as Norman