Hand of Fate 2

The easiest weapon to accomplish this with (that you also happen to get very early in the game) is the Cardinal Blade. It fires a projectile along the ground in all eight directions, each doing 15 damage, and multiples can hit a single enemy if you're close enough.

Round up a small group of weakened enemies and use the ability in the center of them for maximum effectiveness. Regular Skeletons are likely the best to attempt this against, as they have low health to begin with and automatically resurrect over time for re-attempts. Many encounters can be found against them in the Lovers challenge.

Kill two or more enemies with the ability to unlock the achievement.