Both of these are normal type Pokemon so Fighting type moves will do major damage. My Tepig leveled up with a Fighting move so it was no trouble. If you are not so lucky you might want to head to Pinwheel Forest and get the TM94 Rock Smash first.
Once done you get the Basic Badge and TM67 Retaliate. Right after the battle you'll get into a scene where Team Plasma is stealing the Dragon Skull from the museum. Head back up there and watch it all play out. Before you leave Biance will give you the Dowsing MCHN to help you find hidden items. Then head outside where everyone will meet up and you'll be told to here to Pinwheel Forest to find the Dragon Skull. This video is from our unofficial game guide. You can view the full guide here