-Before encounteringSergei, collect everything, especially if you are low on heavy machine gun ammunition. Get ready for a massive fight. Shoot the enemies with the heavy machine gun. After killing them all, Dekker will appear. Use your heavy gun on him to kill him quicker. Ammunition boxes will appear and become collectable.
-Once you enter the room, get all the heavy machine gun ammo. Once you kill all of his guards, restock the heavy machine gun ammunition. Go directly next to him and just shoot him nonstop. Do not let him enter the room he first entered from. If he does, you will be killed by the machine guns in the corners and on the submarine. Also try to aim for his neck. It seems to drain his health more quickly.