[Faceman]

I figure that a Jester or a straight Bard would be most difficult, as they are designed to benefit the whole party with their songs and not so great fighters or archmages anyway. I used to put Haer Daelis in behind and let him sing is song as soon as he got the improved one, although he has some fighting skills.
But at Core Rules difficulty I think it would be very hard to solo the game as a Bard without cheating or using cheap tactics like fake talk, dragon-cloudkill or area changing.