This doesn't go here. *moves*
Anyway, once you enter the cave, you have to throw 3 pester balls at Grimer and it will evolve. Plus, I think if you take pictures of the Grimers in the caves to your left when you first enter, a Grimer will appear very close to you on your right once you enter the next segment of the cave, making it easier to hit it with pester balls.
I think so, anyway. *hasn't played that in a long time*