In my opinion, having a character explain his ability in the middle of a fight (unless his target is unable to continue) is a sign of arrogance on the character's part. You could have him temporarily stun his opponent, and calmly describe his technique while his opponent is stunned.
One potential way to do it, (personal favourite one), is to have it written outside of the dialogue. This enables the reader to understand the ability, without them wondering why the other combatant is allowing the speaker to blab about his techniques.
Alternatively, explain it after the combat to his allies. But make sure he keeps the specifics of it's weakness secret, but allow the reader to know via internal monologue.
In the end I'll be with you.