Try building an rallybot in another file. I think +Str, +Def +Speed +??? +??? will do. Just rally on Donnel. Then pair him up with your strongest unit. Give him stat-boost items and use them. You could also try to forge weapons, or give him one of those weapon-rank increasing books and equip him with a strong weapon. Most importantly; Try to lure the enemies one by one, healing him when needed.