Help!!!

Since you chose mudkip at the start of the game, I am going to assume, May has the grass type. Well, I can't truthfully give you a battle strategy since I don't know what other Pokemon you have, if you do. If you don't, I highly suggest getting some more Pokemon to battle May. Anyways, to defeat her grass type starter, bring out some Pokemon of fire, flying, or bug type to defeat it. I prefer fire if there are any available. It would also be helpful to bring in some super potions and maybe some soda pop to heal you during the battle. So, just grab some more Pokemon available that have super-effective attacks v. The Pokemon she has, bringing some items for healing.

Since you aren't able to get soda pop and fire/flying Pokemon so easily, I recommend you to level up some more your marshtomp and getting some potions and have in your party a dustox/beautifly so you can battle grovyle or treecko and beat it easily. Also use moves like mud slap/shot to lower the foe grovyle's accuracy.

Like pity said above, try to get a few healing items such as potions and soda pops,a good flying pok�mon (I used Beautifly and won on my 2nd try),and then use Mud slap/shot to lower grovyle's accuracy.
It really works! Hope this helps...