There's an easy trick later in the game
Once the Zombies start rushing you, hide in the bathroom. They're Zombies for chrissakes! They are dumb. They try to take a straight line towards you.
If you hide in the bathroom, the zombies entering from the SSW can't get to you because there is some obstacle in the way.
Once I got the TMP, I just chilled by the tub and shot any zombie that managed to get in my line of fire. Then you just pick a quiet time to run out and pickup the ammo (and health if necessary).
The Boss guy WILL DIE ... it just takes about 200+ bulletsd to bring his arse down.