My take on the Huns’ motivation towards the end is a bit of a projection/imagintion (this third act requires a lot of that). But I always took it to be the death rattle revenge assassination; the real loss of the invasion happens in the battle, this is just Shan Yu’s attempt to deliver some pain with what he has left.

Admittedly, it’s not altogether their fault, the Ballad of Mulan doesn’t have much of a finale. But they wanted to narrow it down to a personal confrontation with second act “all is lost and our heroine is defeated and humiliated” atmosphere and, having previously needed to have her win the actual decisive battle against the Huns, it kind of feels like they sort of just jammed whatever was necessary into the film to get there.