What if we used the Eye to set up a kill zone surrounding the safety zone, but leaving about half of the caldera open. Then, when the Waaagh enters the caldera, they find that about halfway into the caldera they get incinerated, but they think they are safe in the other half. Then they spend their time hitting Yar and Zilfin. Finally, when they all enter, then we incinerate them all.
There are two ways to accomplish this. Ideally, the spell works so that we can limit foes that get hit not just by who they are but by where they are (i.e. people in this part of the shadow get hit, but not these.). If that's not possible, we can just quickly and repeatedly reposition the shadow to cover the eastern half of the caldera.
@BoneyM: Is this viable?