The AI usually has predetermined paths. Just remember that the AI tries to dodge the trees as much as possible, unless they plan on ambushing too as it will the most efficient path movement point wise. I consider the ambushing tactic not worth it, especially if you face larger armies. That's also because these maps are heavy on the trees and I rely on my bodyguard cavalry to make most of the kills. Until you take Minas Morgul, running around Ithilien and attack any Mordor army that pops up, big or small, is the best tactic that worked for me.