This achievement is fairly simple to get. Once you reach the point in the game where you must travel to the Lighthouse. You will then be able to select a character who will join your team. Choose Amethyst. Amethyst has a ranged attack that can hit multiple enemies with ease.
Get three enemies to a low health, make sure they are in range within Amethyst's attack, and attack. The achievement will be yours.
If you're having trouble, upgrading Amethyst's attack will increase her range by 25%.