Marshtomp is good for its Attack and its bulkyness. You definitely want Earthquake, Waterfall/Aqua Tail, Rock Slide, and filler. That will make the most of its Attack and be good for coverage. I don't think it can learn Aqua Tail though. But most of its attacks are going to be from TM's/HM's, so good luck on that part.
A good nature would be anything that boosts Attack. So you have a choice with Lonely, Adamant, and Naughty. I wouldn't go with Bold, since that reduces speed. I'd probably go with Lonely/Naughty since that reduces one of the defense stats. For EV's, I like to go with max Attack and max Speed.
Look out for Ferrothorn. That thing will murder you unless you have a fighting move, so if you can get Brick Break or something, get it. Any other Grass-types will kill you as well, but Ferrothorn is the biggest threat since its defense is ridiculously high.