This achievement is earned by defeating 50 enemies while you're in Unlimited Mode.
Unlimited Mode is achieved by attacking enemies and the Star-like bar will increase and once it's fulled you can press LT to activate it.
You'll do more damage, take less damage, Heavy Attacks do not cause you to overheat, and you can just go crazy unless it depletes. Whenever you get the chance just go into it and ATTACK.
You'll earn this through natural progression, if you remember to activate it and earn this 15G achievement!