I got this achievement as a level 2 Jew in the beginning of the game. After the intro sequence (after you leave Cartman's backyard for the first time) you are tasked with finding a few companions. At this point, I went to Skeeter's bar (in the top left corner of the map, hover over the buildings with the cursor and it will tell you what building it is). Talk to skeeter and he will give you a quest to clear the rats out of the basement. Go downstairs, and start a fight with the first rat you see to the right of the bottom of the stairs. I used the new kid's Sling of David ability on the larger rat, and Butters's Hammer of Storms ability on the back "column" of enemies. This should take out both enemies in the back. At this point, the one smaller rat left will attack one of your party members. You just have to block this rat's single attack, then take him out the following round. Hope this helps!