Dwebble. By level 29 it can run stealth rock, rock slide, bug bite, and rock polish or a filler move (it has night slash as an egg move).
I found Shelly fairly easy to beat though. A strategy I used:
i might not have much to offer, so are you really going to trade me a 5iv dwebble? @Rainlove thnkx for the tip
EDIT: i defeated her finally ,