If it's a trap designed, set up, and controlled by psy engineers or something, why not. No comment on the war engine yet, but I do like the idea of the traps. However, I don't think spirits "put their physical resistance up" per se, they just have it. Nowhere in the wiki does it say that they have to think up physical resistance. Just an example, if I saw a ghost and knifed it, I don't think it would have much effect on it even if it didn't see me. So they'd have to have more resistance than just about 75%. I can see where you're going though, considering the fact that Luca always sees the enemy before she gets hit by a spear.