Damian is very capable. He's not even going to belittle himself by adding 'for his age', he is capable for any age. He has been trained by the League of Assassins. He is Robin. He is the Son of the Bat and Heir to the Demon. He is very capable.
And it isn't bragging to state that he is possibly the... well, alright. Third, most-capable combatant of his family. He is not so arrogant as to believe he has surpassed his father (yet), and even Father admits that Cain is the strongest fighter. But in this case, third is still very good.
The point here is, he is not afraid to duel you, and frankly he is offended that you keep implying that he should be. In fact, he's starting to think you must be afraid to duel him. It's training, everyone in the family spars for exercise and practice - and this is how he knows he is third in the family in terms of skill. And he intends to maintain that ranking.
And. Alright. Yes. You have superpowers. Of which he has yet to be fully informed of the particulars. But he is not afraid of metahuman abilities. He has seen them before. He is not intimidated by whatever unnatural abilities you might have. In fact, he might even go so far as to say that if you need such abilities to defeat him in a spar, you are less capable and having to resort to cheating to win. ...And then Jon would ask him if he thought it was cheating that he had powers and Damian would have to retract his statement. So he won't say that. Out loud. But he does intend to get you into a spar.
"You have dueled with Father already. And Grayson. I see no reason why I should not also give you instruction." Because that's what it would be, ultimately, right? Teaching. Dueling a more capable combatant is how you learn. This isn't just him being proud or feeling he has to prove something. He's offering you a favor.
...And he's not scared of superpowers, okay?
