[Edi]

That's what the glamour ability does. Glamour units (even without stealth) cannot be detected by any means, so if you have a bunch of glamour units defending, they will precisely jump your enemy from behind the trees and stick him full of holes.

This makes the EA nations of Vaheim, Helheim and Tir na'Nog so very painful, since they have tons of glamour units, some of which are tough enough on their own. Someone who plays well with their raiding capabilities will cause you to rip out your hair...