I think the monk approach is much better here
Problem is that you're not allowed to convert anything, use any items, or worship any gods, so you can't mitigate his attack penalties. That's just a total disqualifier (edit: Avatar beat me to it)
Or what about the rogue? He increases the boosters you find (the only ressource you got), maybe that makes him strong enough?^^
Do you mean the thief? Unfortunately it's not as helpful as it sounds. The extra potions and mana boosters will be wasted, and his potion power class feature is of no benefit. So you're just getting one extra attack and health booster.
I think good choices may be Berserker or Bloodmage (Gorgon might work, but she'd block you off), but you'd still need to get an absurd amount of bonus XP to pull this off without the Fighter (who is probably too weak). I'm not sure if this is doable at all, but I'd love to be proven wrong.