The little twat can't be beat. No matter what. After you spend 20 hours clearing out the first battle in the tunnel, kill the Gold Elite, right as you turn that corner- you die. No buts about it. You can throw as many grenades around the corner as you like, they won't kill him. Above all, don't even try to quickly turn the corner and BR him. He hits the INSTANT you turn it.
Just though I'd share that.