Is it the one in Giddeus? I remember trying it 3 times, nearly won the third time but that damn Crawler kept chain casting Cure3 on himself. I looked it up and I don't know the strategy for 6 people (it would be easier) but basically you need to go in with damage dealres (ie. BLM, BLM BLM or DRK, RNG, BLM, or DRK, RNG, SMN). That will garantee a win, although you may want to check up the strategies.