Not hard at all...

Start the game.
Get enough money to open he door on the left (its on the left when you have your back to the PAP Machine)
Get enough money to open the next door.
Get enough money to clear the debri
Go over the bridge.
Get enough money to open the Z-B Teleporter (I think it's Z-B).
Save one last zombie (Can do this on round 2)
The 'room' between the bridge and the other room is where you wanna kill the zombie, it's the part with the air con box or whatever it is, it's got a letter in it, you'll know it.
You wanna kill your last zombie in that room.
Once you've killed him you wanna run to the teleporter, inside it and go directly in the middle and crouch then turn to face the front.
If you've done it correctly then it will work because i'm doing it right now.

Been going for an hour 21 minutes and im on 71 I'm leaving it on all day though to hopefully get to round to 1001
You can kill the dogs if you wanted to but I wouldn't recommend it.
Just plug in your controller if you can and go outside, sleep, eat, just do something but don't sit there watching it because it will seem like it's taking literally forever.


Hope his helps


Cheers.