User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20160721023809/@comment-24.113.252.213-20160721053219

But I never meant to say that he's doing the fighting himself. I meant that his style of fighting is translating into his "Team Battle" tactics.

He's having the fighters of his peerage attack the enemy, and try to overwhelm them the same way Issei does in his fights, while having his healers (Asia and Ravel) hang back and heal the team's injuries so they can stay in the fight.

He might even be using this strategy: He has his peerage keep the enemy peerage busy while he takes on the enemy King. This way he can end the fight quickly, and his teammates get the chance to fight. While this strategy is effective, it is very sumplistic, and easily countered. So Issei is likely getting low scores because his tactics are too simple, and he's getting countered too often. So even if he wins, it's not a "clean" victory.