Here's a Simple and Easy Trick to get this Achievement.
1: Go to Play in Main Menu
2: to Challenge
3:
A) Difficulty = Easy
B) Boss A = Kitty Chopper (or any easy boss for you)
C) Boss B = None
Boss C = None
B) Arena = any arena you want it's doesn't matter
4: Press Play and Defeat the Boss You Selected and Here We Go You get the Achievement
