Maybe it was a glitch and they've since patched it, but when I got to the last boss, I threw down my turret and literally stood perfectly still directly in front of him and unloaded with my combat rifle. I don't even know if he moved to attack me. I was pretty disappointed, frankly.
But then on the second go he was just retarded. I ended up throwing myself off the cliff every time my health got low, because if he kills you and you respawn, he gets all his life back, but if you fall to your death and respawn, he gets nothing.
Super cheap, but effective.