I believe because of their mobility you could treat them like a surgical scalpel. 2 units of 20 I think will wreck havoc on most things.
I think a list like this could potentially have a strong psychological effect on opponents also. If the edges of the board are not safe because tree revenants can get there at anytime they might funnel their forces to the center of the board, where your woods could be waiting to embrace them. This is conjecture of course.
I imagine a list with Cogs and alarielle could be very effective. I have to run the numbers though.