Hrm. Technically, such a feat shouldn't be possible since you're required (except when story line interferes) to have three members in your battle party at all times. However, if you're willing to let everyone participate in at least one battle, don't do anything except keep Crono healed with items and let the rest of your party die, then simply don't revive them for the rest of the game. I'd imagine this would make the Magus battle pretty difficult with only a single element and (if not using Frog) no Masamune, but hey, everybody loves a challenge.
Then again, if you want to be cheap and partially retarded like me, create a New Game + and defeat Lavos the minute you gain control of Crono. Voilą!
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What?
