Hmmm... Is it a glitch?
I'm sorry for the low rating, but I can't see how it is possible to finish this.
I was left with one large zombie, none others, and I managed to put about 75 shotgun blasts and over 100 9mm's into it, and it still didn't die.
Normally it wouldn't be too much of problem, because in most decent zombie survival games, you can move just faster than the zombies.
This is also known as a 'realistic' moving speed.
Then you could just circle it with enough skill and take it out over time.
But no, here you have a large zombie who moves faster than you, and can somehow survive being more hole-filled than a pepper pot.
I'm sorry if this is just a bug or something, but I can't say I enjoyed this game, I found it getting monotonous before I even got the Steyr.
I'd find it better if you could move at a more realistic pace, not get stuck in the middle of the room (This is how I died the first time, an invisible wall in my way, it seemed), and actually kill the last zombie (or I assume it's the last) with a reasonable number of shots.