What does Gerrant Merc's second skill do?

[windhelmguard1]

Attached an image. I dont understand what it means by damage +30%? My stats did not change.
Does it mean I do 30% more damage to monsters? If so, must i stand behind gerrant for this to happen (else no more bonus if im far away)?

[Hanakarasu]

Not exactly. What the skill does is that it allows Gerrant to apply a debuff to an enemy, which makes them take 30% more damage from attacks.

[windhelmguard1]

How do i know which enemy he applied a debuff to? Are enemies in front of his skill debuffed?