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Keep shroomish until it gets spore, then let it evolve (this is around lvl. 48 or so). That way you can use spore+focus punch with Breloom to own things. You should get an electric pokemon (I'd recommend Manectric) to beat Glacia, she'd be a big problem for you with that team. Slakoth sucks, I'd...
Shedinja@Leftovers
Bold Nature (+Def, -Atk)
-Sandstorm
-Rest
-Harden
-Substitute
Owns you.
And btw, e4protector, Umbreon can't do crap offensively. It should only be used as a staller. You suck if you think differently.
The first one. Skarm doesn't have much that could hurt Gross. But if I had a Magneton (which I currently don't) I would definetly use it to own skarm in this, no, ANY situation.