Best level I found to do this was clearing rat's from the basement of Skeeter's Bar. A small side quest.
Let Butter's get knocked out in each fight and continue fighting and killing the rats. There are 3 battles in this basement and are quick and easy.
Let him get knocked out and stay out in battle, defeat all 3.
You can even use environmental elements to stun some of the rats before battle making it easier for you.
Will void "No Child Left Behind" achievement.