Fantina

General Info - Fantina uses the Ghost type, so that should automatically tell you that Normal and Fighting types will be virtually useless in this battle. Ghost has two weaknesses, Ghost and Dark. You probably have some sort of Pokemon with Bite or another Dark move simmilar to that, so bring it along. If you have been training a Ghost type and it is a level or two above Fantina's (so you know it is faster) that is an excellent choice.

Lv 32 (Ghost/Flying) - Driftblim has a lot of weaknesses because of it's Flying type, so if you don't have any Ghost or Dark types, attack with Electric, Rock, or maybe Ice. Driftblim has a lot of HP, but with that many weaknesses it should go down quickly.

Lv34 (Ghost/Poison) - Gengar can be annoying because of its high speed and special attack, but if you have a faster Psychic type, like Kadabra/Alakazam, Gengar's poor defences will be it's downfall.

Lv36 (Ghost) - Mismagius is basically the same thing as Gengar, with fewer weaknesses. Either attack it at its weakpoints or withstand it's attacks a few turns and take it down with normal damage moves.