In the Lovers challenge, you'll be given an artefact called Holy Purification. In the first fight you come across with five or more bunched up skeletons at the beginning (this happened to be the first encounter for me after getting the artefact), immediately hit RB to slow time and aim at the enemies in the front to account for their movement as you throw.
The artefact should kill all five or more instantly and also unlock the Focused achievement.