Very simple achievment to obtain. If you can at least think a small bit. He will use skills that enhance him, heal him, and the other one that works as a push back (IDK the name of it.) Before you start the battle, I would suggest switching to heal, Push back, stealth (my fav), and any other of your choosing. Walk to the tree to start the battle. The first thing I did was use my assault rifle, then, when he was injured, he would heal. Stop him from healing by using the push back skill. It wall cause his healing skill to fail, leaving him still low on health and giving you the opprotunity to strike him more. If (and you should) you still have ammo for the assault rifle, keep shooting with that. If he does heal and you knock him down afterwards, use your stealth to sneak up on him. You should use the shotgun for this part. It should hopefully either stagger him, or knock him down ( haven't played it in a while ). Remember if you get low on health, use the heal skill. You should have him killed in at least a couple to a few minutes.