Well even if Cresselia was death fodder, I still would have continued to (or at least try to) sweep. What else did you have in your party besides the CS Heatran? Not to sound arrogant or anything, but I had a +4 Spe Empoleon behind a sub. I think I would have been able to take out the Heatran, and if not, I had 3 other Pokemon to try to knock it out.
Like I said, not trying to sound cocky or arrogant, just saying that I had absolutely no reason to DC.