Like JapaneseWallpaper said, you should level up the rest of your mons to around Level 30ish.
I would recommend picking up either Pachirisu/Emolga, Phanpy, Bunelby. If you really are willing to work to get it, you can even pick up Trapinch from the Game Corner, but it does require a somewhat large amount of time/coins to invest into it.
Anyways, make sure you watch out for his Crobat especially. Mareepy, Pachirisu, and Emolga can take it on decently but it can sweep you if you aren't careful. Other than that, the rest of the mons should be easy.