To beat it with just Crono... I don't remember how buff you need him to be, but if you do it you get the special programmer's ending. You have to start a New Game + and when you go to the fair (this is all from memory), go to where Lucca's portals are. Yuo will have Marle, so she must get sent back to the Middle Ages first. It seems to me that there will be a little shiny thing somewhere in the original portal area (grrrr... can't remember). Go in there with just Crono and it'll take you to Lavos. I think you have to do all the stages of Lavos (outside, inside, true Lavos).