For the record, I just wanted to add that if you didn't do this when you were running through the ladder with all the characters, you CAN do this in Versus mode with 2 controllers.
The fastest way to go through everyone is load up Versus Two-On-Two (Tag), select 2 characters you need to complete on controller 1, and whoever on controller 2.
At the loading screen, on controller 2 only, if you press X twice and Y twice it will make player 2 start at Quarter Health.
In the match, do your neutral/back throw, and your forward throw, punch once (this will kill Player 2's first character), tag out, do your 2nd character's throws, and then finish player 2's 2nd character. Rinse and repeat till you have the achievement.