This guy can be absolutely clutch in a Silvan build, particularly when paired up with Thranduil and enjoying the boost from Celeborn. The trick is waiting for the strategic moment, which is made all the easier by Thranduil's combat action. I've lost track of the number of times that multiple enemies have bounced off of Greenwood Defender, allowing the other Silvans to stay ready to counter-attack and get the board under control. Even if he falls to enemy attacks, oftentimes the ability to block a few smaller enemies first is invaluable.
If you aren't running Thranduil, you can always pop him in with Sneak Attack during the combat phase (less cool than Gandalf, but potentially more effective under the right circumstances). If he lives through his first round, he can soak up archery quite nicely (especially for a Silvan) and then be healed up by Silvan Tracker to repeat the process. The only thing he's really missing is sentinel, which would elevate him to top-tier in my opinion.
Great card for Silvans that has almost completely replaced Defender of the Naith for me.