This one's simpler than I expected, actually got it in about two minutes.
First off, change the difficulty to Easy. Start a 1v1 cage match, doesn't really matter who you go with but I'd recommend high overall for yourself and as low as you can find for your opponent.
Once you get into the match you're gonna want to let your opponent beat on you, at some point they'll knock you down and they'll begin climbing the cage. From this point you'll wanna press
to start climbing the cage, on the same wall as them but far enough away that you won't just drag them down. You'll probably be a bit behind them once you make it to the top of the cage, they'll most likely be attempting to climb down the other side. Shimmy up within striking distance and simply press
and you'll grapple and hit them. Do it 3 times and you'll automatically throw them back inside, and the achievement should pop.
Pretty painless 15G