Jill Zombie will be the Final BOSS of the game! then Chris will salute her at her grave and then and put a gun into his mouth 

Exploding barrel and the sniper rifle are your best friend. Before you meet with the chainsaw guy (CSG), don't shoot at those exploding barrel scattered about. When he appears, move away from him and shoot at the exploding barrel when he is near them but don't stand too far away. When he is down, go near him and give him a good punch in the face.

Then press RT + A to make a quick 180 turn and lure him towards the next barrel. Rinse and repeat this until there are no more barrels lying about.

Now you will need to rely on the environment. Remember the building that you helped Sheva jumped acrossed? There's an alley beside this building that leads to the backdoor of the building. So the trick is to lure him, going around in a circle between the building and the alley.

Now, run to the end of the alley but don't turn into the corner leading into the building yet, instead, take up your sniper and headshot him when he is moving towards you. Then run towards him and give him another good whack in the face. Then run away and go around the building again. Rinse and repeat till he goes down for good.

Make sure you turn your map ON to keep track of his location and BEST to reload when he is still moving in the building and you are near the end of the alley but don't move too far away from him cause he will backtrack.

To be safe, always do the Headshot + Punch combo once. Then lure him around the building again. It's good to be on the move cause there might be zombies that are coming close to you. If they grab a hold of you, that means precious seconds are going to waste while the CSG is getting closer and closer.

About the zombies, I will always ignore and run past them and pray Sheva kills them off for me

And most importantly, stay calm. It's fun punching him