When fighting the ice beast (at the halfway point of the Old Town level - the final level accessed from the level hub), he will conjure large "ice meteors" from the sky. He will often summon six in immediate succession, and they are some of the slowest, easiest to hit projectiles in the game. Be aware that around half health, he will switch to phase two and stop summoning the meteors, but even playing aggressively, there's a good chance you'll unlock this naturally if you hit all of them back at him during phase 1.
Don't forget you can restart from the last checkpoint to attempt this as many times as you'd like. The achievement popped for me mid-fight, so you should know before the fight ends if you need to retry.
Good luck!