Steven Universe: Save the Light

The easiest way to unlock this achievement is to find a very strong enemy that can be affected by Steven's shield bash attack and knock them off the fighting area. This will take away the remainder of their health, and if they have over 100, unlock the achievement.

The other way (and likely the way the game expects you to do it based on the achievement art) is using Garnet with her weapon fully upgraded. She will be a heavy hitter, and if you use a Redo Charm to reset her skill points and put everything into attack and luck, she should get a critical hit for over 100 in no time.