Maybe try overlevelling?
This could be one way of defeating Grovyle. Because when you levels up, your stats will increase, which then means you may either live a Mega Drain (or whatever grass attack it has), or outspeed and get to attack it first.
You could also try to Hax it to death.
You could also try Spamming a Move like Mud Slap if you still have it, to lower it's accuracy so Marstomp won't get hit, while lowering it's health at the same time.
This'd also work with overlevelling too, since then you wouldn't get hit by Grovyle, and if you did, Marstomp wouldn't get KO'd anyway.
Or try EV training, so you can take a Grass Move without getting OHKO'd.
I'm not sure how many EV's in each stat you'd need to take a Grass Move, but maybe Marshtomp will have an even better chance of Living if you give it as many EV's as possible in the two stats. (Probably HP and Special Defence)
I hope I helped, in some way!
