I didn't know until that point you actually had to kill the hearts to win.
I also just found out that if there is actually nobody there, rounds are lost pretty quickly one after the other. I was tempted to let it keep going. The standard for defeat is a fair bit lower than the standard for victory, apparently.
It appears two strikes kill any heart at any level, but you get way more experience at higher levels where they have more health. I kind of doubt that was intended.