Go onto Route 4 and catch a Darumaka. It'll be able to back up your Pansear. While you're on Route 4, get a Sandile as well (for Elesa's Gym).
Have your Dewott deal with Dwebble, since its Rock typing means that it's not weak to Fire, and has a super effective move against your Fire types.
I know that Fire is x4 against Leavanny, but his Leavanny is at L23, so you need to do some training before you beat Burgh. I would recommend your Pansear and Darumaka are at least L20 before you challenge Burgh again.