Earthquake
Surf
Ice Beam
Rock Slide/Body Slam/Brick Break/Counter/Rest/Protect
Earthquake is the best move Swampert can learn, due to STAB and its high ATK. Surf is for being a standard Sp.Atk move w/STAB. Ice Beam slightly covers the Grass quadruple weakness, due to its averga but not high Sp.Atk.
While Rock Slide, Body Slam, and Brick Break are simply good Atk moves, Counter punishes foes with even stronger Atk moves, assuming Swampert didn't faint. Otherwise, Swamper can use Rest while holding a Chesto/Lum Berry, or it can hold Leftovers while using Protect.
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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.
There is actually a few things you can do after beating the elite four on Pokemon Emerald Version. First, you should go all around Hoenn and listen to the people who have heard of your outstanding achievement of defeating the Elite Four the the Champion. Then, the next time you see Scott, he will bring you to a place called the Battle Frontier. There are 14 more badges to get over here (or is it 16?). WARNING!! Defeating the Battle Frontier will take you an extremely long time and WILL be frustrating. Just keep on battling and you will do it.
You can't get any of the Hoenn starters Sceptile (Treecko), Blaziken (Torchic), or Swampert (Mudkip) in FireRed/LeafGreen; you'd need to trade from Ruby/Sapphire/Emerald. If you do trade a Sceptile, Blaziken, or Swampert (or one of its previous forms) from RSE, you should probably mate that Pokémon with a Ditto in your RSE game first. After you hatch the egg in Ruby/Sapphire/Emerald, you can either trade that baby starter or the original higher-level Pokémon to your FireRed/LeafGreen. That way, you have one form of the starter in one game and another form of the same starter in the other game.
Earthquake Flamethrower Rock Slide Take Down/Hyper Beam OR Earthquake Flamethrower Sunny Day Solarbeam The best thing about Camerupt is its high Atk and Sp.Atk, so you should take advantage of that if you choose to use Camerupt. However, with its low defenses and Speed, it can easily faint to its quadruple weakness Water, so Camerupt isn't recommended for use at the Battle Frontier.
At the Battle Fronitier a house with 2 old ladies. One of them should teach you thunderpunch for the right amount of Battle Points.