Hmm... That's strange... I played on this map against Daleb, and she had machines on expert difficulty. I defeated her ballista with 111 gremlins. As far as I know, the ability affects the ballista in the same way as it affects a tent... Is this a bug?
Maybe the hero had "Endurance" (+2 to hp) skill upgraded?
And also, you can reduce the hit points of war machines (for example, by "accidentally" hitting them with an area-of-effect spell). This is also taken into account.:smile19: