I had so many problems with getting this one to unlock.
The idea is that you rocket punch an enemy into another causing both to explode.
I had destroyed multiple Phasm enemies with one punch throughout my play through but just wouldn't unlock.
I finally got it to unlock on the third wave of the Malibu Combat Challenge by punching a Skull enemy into i believe another Skull however my vision was slightly obscured so could have been a Phasm. If your having issues with hitting Phasms into each other and them not unlocking then try focusing on single punching the Skull enemies until their health is low and trying to deliver the final punch with another enemy behind them.