Plague of Death (3rd chapter)+7 mana treasure murloc spell(murky’s battle horn)=visual draw(both characters explode) but you get to fight next boss.
Doublechecked it with 1 minion on the board. Spell is generated by Flo (Brann’s treasure).
Known Bug:
Did you get to pick new cards after this fight? My bet is no, since both heroes dying results in a Draw, not a Win, and a Draw will not end your run, it will just make you face another boss of the same level.