Wait until a zombie drops the "Death Machine" perk while making sure you're near George when you pick it up. Shoot at him as much as possible. Since the Death Machine has unlimited ammo for a short time, this will deal significant damage to Romero. Even if you pull him into the water and he returns to normal, this will still inflict damage. It will require a massive 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.