How to kill an untargetable flagellant?

So I just pretty much "won" a battle(Lost all my stuff except my vargheist). However, there's a single untargetable flagellant unit left. Since flagellants are unbreakable he will never flee. How do I win this battle without doing it all over again? I know it's a pretty serious loss to lose all your units, but since I'm playing the vampire counts a fair amount of them will just come back if I win.

Please help, and good afternoon.

Background lore: I'm playing mannfred, and just wanted to take out volkmar early on, this was a battle against volkmar and his army at turn 5 I think.