You can easily defeat Greywolf on the roof of his hideout by using Proximity Mines. Just place one near where you enter the roof, and as he circles around he will set it off taking a large amount of life. Just be careful not to set it off yourself. When your mines are down to one, start sneaking behind him then take a few shots and head for cover. Use the one mine you saved for when he gets low on life, because he will back himself into the gap in the corner directly across from the entrance and stay there shooting nonstop. Place a mine in that area beforehand and when he goes there he will set it off, dealing the final blow.