When you visit Mephisto's Realm for the first time. you'd need to do a series of chores to light five candles. In one of the puzzle areas, you're required to toss leapers into a pond of fire. This is probably the easiest place to grind this achievement.
First off, kill off three of your characters (don't worry, they can be revived at an extraction point later) so that the AI won't interfere with what you're doing. Next, go to the point in the following video - it's just to the left of the pulpit when you enter.
A leaper will spawn indefinitely, and you can press
to grab it when having your back to the ledge, and then press
to toss it backwards into the abyss. Rinse and repeat. Once you get a rhythm, you'll be easily throwing an enemy off the ledge every few seconds.
You'll unlock the following achievements in the process: