I've challenged Marlon once so far and lost(I reseted the game though, defiantly going to try and not do that on White 2...). I used Emboar, Golduck, and Heracross(Level 50 for the first two, level 49 for Heracross). I'm not really in the best position in the gym, type wise, as four of my Pokemon are weak to Water type moves. I want to use Golduck to finish Marlon off but it probably wont work out too well, it only has Psychic and Surf for damaging attacks(Plus Disable and Flash).
I can't wait to battle the Gym leaders on Challenge Mode.