Single Out

Single Out is a passive skill, unlocked at level 68.

In-game
This skill is specifically designed for boss fights in general, though it really shines against bosses like Urzael, due to the fact that some of those foes prefer to fight alone. This skill may be worth the sacrifice of another passive skill for these cases. For bosses like Azmodan, which spend most of the fight accompanied by minions, it is slightly less useful, but still one of the most effective abilities.

One may also try to use it in tandem with Slow and Immobilize effects in attempt to separate powerful foes from each other, but it's a far less viable strategy.