Wait until a zombie drops the "Death Machine" perk and be near George when you pick it up. Shoot at him as much as possible. Because the Death Machine has unlimited ammunition for a short time, this will deal a great deal of damage to Romero. Even if you pull him into the water and he returns to normal, this will still cause damage. It will require a very large amount of damage to defeat him. You do not necessarily need to turn the power on or kill him near the water; just as long as you keep doing damage to him, any weapon will bring him down. You will notice that his light hammer turns from blue to orange when you are close to defeating him.