In order to unlock this achievement, the first thing you need to do is load up a Klassic Tower.
This can be found under ONE PLAYER --> TOWERS --> TRADITIONAL TOWERS
Once there select the KLASSIC TOWER and choose LEATHERFACE as your fighter. (Variant does not matter for this achievement.) You may also choose whatever difficulty you like. Now you're going to want to play (or restart the tower) until you get the PIT STAGE. (If you're unfamiliar with this stage, it's a long bridge, set at night, with characters fighting on another bridge in the background.)
When you've gotten this stage you need to win and at the FINISH HIM / HER screen you need to perform LEATHERFACE'S STAGE FATALITY. This is done by getting to CLOSE RANGE (right next to your opponent) and inputting Forward, Back, Down, Up.
If done correctly, you'll uppercut your opponent into the pit and the achievement will pop. Nice and easy!