I used C(h)rono, Marle, and Magus, and I had Marle heal and Chrono and Magus use Luminaire and Dark Matter. It doesn't really matter what party you use for the final battle, at least as far as what ending you get goes. *nod*

I don't remember what my level was, but I didn't think it was that high, and I didn't have any trouble. As soon as I'd completed the battle and watched the ending, I immediately took on Lavos with Chrono and Marle alone in New Game+ and again emerged exceedingly victorious. That ending rocked.