I'd use a black hybrid lower than level 10. I used my level 9 Dark Angel but only as an experiment. I'd say try something like a Night, Archangel, Siamese, Goblin or something exotic like Cursed Rose or any black tournament boss like Gatekeeper, Avalanche or Villainess.
Also try not to use a dragon that's more than 10 levels above an opponent. I found more losses that way.