Well, I did it. All my guys were around level 31, with about 350 hp each, and 4 charges of Nuke a pop. I have to say, though, that a lot of it was due to luck.

The first couple of levels are definitely the hardest, as they are for anyone. Phantom was a breeze. I used Fire 3 vs. Lich and took him down in a couple of turns... he did use Nuke, but everyone survived, even the guy without a ribbon. I actually had better luck with Kary the second time around than with the first: she kept using fire2, which did next to nothing. Kraken was similar, in that he used lit 2 and dark. Tiamat was actually the easiest: I couldn't use Brak/bane, so I actually used nuke on him, 3 shots of which were enough to take him down in a single turn, before he even made a move.

By the time I got to chaos I was completely out of potions, but everyone was at full hp. Nuke vs. him did anywhere from 100 to 400 damage on him. As you can probably imagine the issue wasn't the damage dealt to him, but my party being able to survive his attacks. It was definitely close: one guy lived with only 63 hp left. But I won, and that's all that matters.