I dunno if it's my bad luck or something but even after getting my Mons to lvl 20, i'm still defeated. Corsola's Bubbles hit 4 times (I'm sure that's not even a thing) and everyone else keeps getting crits. It's ridiculous, I went the gastly route and it dies very easily to acrobatics 

Huh. I was under the assumption that the Gym Battles were designated as Boss battles, which meant you couldn't have Gastly possess something of Roxanne's. I know that you can possess Leaf's Bulbasaur during the second battle near Viridian (did not possess Bulbasaur in her room because why would I want to pass up the chance to mess with her~?) but I guess it was kind of hard to gauge.