This achievement can be obtained while working towards Genius Economic Bad Rat achievement. Based on the guide for that achievement, here are the levels that can be completed "easily" using only one rat:

Level 1 with 1 rat: Cannon Rat (rotation=38, timer=9)
Level 2 with 1 rat: Cannon Rat (rotation=0, timer=1)
Level 4 with 1 rat: Batter Rat (rotation=34)