A Small Transformation - 40G (Original Achievement)
To get this Achievement, you must be attacked by a specific Hammer type attack that only Amy uses.
So you need to get to Stage 2 in Arcade mode and let Amy beat up on you until she uses it. I changed the Difficulty to "Hardest" and sat there until she used it, if she doesnt and you are defeated, you will get the chance to Continue and try again so do so.
Amy's correct attack will start with a short scene where she points behind you and then smashes you with her Hammer after you are distracted. The Achievement will unlock right after you are hit.
You can also get this in Offline Versus by having your 2nd Controller play as Amy and use the attack on you (Down + A). This may be quicker assuming you DO have a 2nd Controller.