Easiest and probably quickest way of getting this achievement is by playing Ozanius in Quick Battle.
Go into Quick Battle, select Hilde or Nightmare.
Press X at the menu to input search criteria.
Search Region as Others and AI strength as Intermediate.
Keep searching til you find Ozanius. Only takes a few seconds.
At the start of the match, hold back, until you reach the edge of the arena. Ozanius will chase you and tends to stop infront of you, blocking.
Use your characters Left Trigger throw to send him behind you and off the stage.
Takes about 4 seconds per round. Rinse and repeat.
Also a great method for boosting your XP and Level.