"While he's concentrating on his magic we will take the enemy general's head."
Uhm, yeah. Sure. You go ahead and try that, old man. Lemme know how it goes.
I can hardly tell what's going on because the way this battle has been portrayed is such a mess. But this is exactly the kind of thing I would do to bait my enemy into overextending themselves and putting them into a vulnerable position. I'm hoping that he's got a small, elite unit tucked away somewhere that the invading army passed without noticing. With him distracting them with his flashy magic, they won't see such a unit popping up behind them to take their general's head.