Firstly it's Damage.
Secondly, it's his ability. Damage = Damage Opponent means he copies what's written as damage on the opposing card. If he's against Deea, his base damage is equal to Deea's 5, and his bonus goes to 7 total.
So in order to OHKO with him, you need to either go against Stop opp. Ability, or face another 8 damage card, or face a 7 damage card if Timber is present in your team.