Easy victory through game

Start with a Mudkip. throughout the game, only level it up. when starting, before you even challenge the first gym leader, level your Mudkip to level 16 (or higher) until it evolves. Then challenge the leader. Throughout the game, challenge all the trainers you meet and defeat them only with Marshtomp/ Swampert.

As for grass types, learn Ice Beam (tm13) or BLizzard (Tm14 I think. correct me if im wrong). Usually, you can KO a grass type if you're at a high level. If not, capture a fire type to assist you, preferably a high leveled one, or one with high Sp. Attack Stats (i didnt need this, so im not sure which one to capture).

The bad thing about this strategy is that if your rival is able to launch his attack first against your Swampert, it does 2x the normal damage, so you may want to fight your rival's grovile/sceptile with the fire pokemon.