Big Joe (XG) vs Hetzel (FE10)
You need insanely bad defense for Hetzel not to tink - around "takes 150% average damage or worse" level defense. Flip side, Big Joe himself probably tinks, at least since I'm willing to give Hetzel terrain credit to his defense. So... this fight lasts forever. Neither the boss-wins tiebreak nor the damage-dealt tiebreak work, since they're both bosses and deal 0 damage total. In conclusion, both lose. Epic.