Dummy Account Solution
Plug in a controller to your computer. We will use this as the main account. Go to Options then Gameplay Settings. Change the Rounds to Win to 1 instead of 2 to speed up the process.
Go to Multiplayer then Versus. Make sure to click on versus with your controller because we will be doing this with the controller. Pick a large character (either Cyborg or Swamp Thing will do) with your controller. hit the Enter button on your keyboard and this will be your second account (dummy account). Pick anyone with this account.
In the match, you will start at the red bar so use your controller account to bring the second account down to as little health as you can WITHOUT killing them. Get over to the hanging crocodile's tail. Make sure you are on the left side of the opponent and hit
to finish your opponent off with the crocodile.Without a Dummy Account
Go into single player, then single fight. Pick Cyborg or Swamp Thing for yourself and the CPU as anyone. Bring your opponent down to very low health WITHOUT killing them. Get over to the hanging crocodile's tail. Make sure you are on the left side of the opponent and hit " ; " to finish your opponent off with the crocodile.Achievement Pop!
Note: It might take a minute for the achievement to pop.