It's done.
By coincidence I found a way to kill the leader at the right time by copying a spell from another mod. Don't know how it works but 5 castings were ok so I suppose it's always.
Erase Spell is Thau 7, Astral 4, costs 9 pearls and removes one level of horror mark. Cast it until the icon is gone. Casting it more often will probably have the effect that horrors avoid the caster.
If someone really intends to use it keep in mind that during the battle only one unit should be there (no guards, no autosummons) and that should not get close to the dragonflies (because they won't do their ranged attacks anymore) during the first round nor kill them by any means. After the first round the unit is paralyzed so placing it front and setting it to (hold)(attack) should be fine. Maybe there's some precision issues (don't know how to give a weapon 100 prec) so front is better than rear.
And back your turns up in case it screws something up but it shouldn't.