>>14578955Chesnaught might not fare too well with fire and ice, unless its level compensates the speed.
Get ice/fairy and ground coverage more. Really, the Elite 4 aren't that hard assuming you have hard hitting moves that can one hit or two hit KO, as well as revives and potions. I pretty much went through the E4 using a strong Fairy attack, Thunderbolt, Brick Break, Mud Shot, and Shadow Punch.
I think Toxicroak would be pretty good. Bro helped me a lot, though I think Yvetal can cover the dark (and maybe thunder) grounds.