I had planned to use my Arcainine to plow through his gym; however, in the quest to get him his best moves as a Growlithe I had to keep him 5 levels higher with a Eviolite attached to give him the stopping/staying power to do so. By the time I evolved him he had Extremespeed, Outrage and Flare Blitz. I planned on letting him use Outrage and sweep the other team but decided to let the others catch up.
What I did end up using was Lucario with Dual Chop or Dragon Pulse (honestly don't remember which) to get through the gym. The steel typing might seem like he could take a hit but, sadly, even resisting those beasts his defenses aren't up to snuff so I let him lead and tear through whatever he was faster than.
Druddigon is a good bet as a dragon type; however, his speed is to low to really take advantage of his awesomeness in this gym. He'll probably end up taking extra hits and even with his decent bulk he wont last long taking those kind of shots. If you provide him with Trick Room support he's golden but I don't think that's available at that point of the game.