I just beat the eight gym. Current pokemon list:
Serperior - lv.50
Victini - lv. 45
Simipour - lv. 43
Gurdurr - lv. 41
Leavanny - lv. 30
Tranquill - lv. 21 (only because it can use Fly)
HMs are not needed that much in this game, I am surprised. The most I've used have been Fly and Strength (Surf only a once or twice).