Morelull

I love the concept of this Pokemon, and the applications it offers to a Grass player. If you park Morelull behind cover someplace, it'll start producing Wither effects on subsequent rounds, cutting into the defense of whatever's nearby, and anything that gets too close can be put to sleep with Spore to be dealt with through allied attacks.

Of course, Morelull is easy to hit and slow to move, so actually setting it up can be tricky... but that's why you run, oh, I dunno, seventeen of them and pollen-landmine the whole arena :-D