You will gain way too many levels just from boss fights if you attempt only 1 character. The experience that is initailly split between 3 characters now all goes straight to one character making that character level up faster and go farther in the level gaining pyrimid.

If you could use all 3 characters I would say it probably wouldn't be all that hard but what about those bosses that hit you with say confuse or death?

I am not saying it is impossible....just insanely unlikly.