(Too much stuff in previous post to put into this post)
All that stuff sounds really good, except the end. Maybe insted of staying as a tree, the artifact would keep him from fading away or changing form, preferably after he had turned into something really powerful, like the super-trow. Then, you would have to throw all your forces at him, and maybe if you were lucky you would kill him with a couple of your units left.
Also, it sounds like you would need a lot of reinforcements throughout the level. If the faceless man appeared in the first group, you would wonder if he was hiding in every group of reinforcements that followed, adding to the "suspense" of the level.
