This question has been asked many times and answered many times.
First, after getting the gun, forget about it, don't use it before the boss. Just collect the ammo. When you get to the boss, you should have 9 stores, if there are fewer of them, then you missed somewhere.
Secondly, when fighting the boss, try to save ammo. In the first stage, beat the boss to the punch. Approach him and when the attack animation starts, immediately step back. At this moment, shoot at him with a pistol. When the boss falls, come up and start hitting him with a stick. In the second stage, try to get rid of ordinary zombies with a stick first. Then keep baiting the boss to attack and at that moment shoot at the red sphere. When the boss falls, hit him with a stick.
I found a video of a boss fight on YouTube