Oh... it has been a few months since I played this but it is the best slow game ever.

Try to get the zombies as they are coming out of the ground since you will kill them with one shot, shoot the smaller ones in their knees and they will fall over, don't waste your shotgun at long range, use breaks in the "action" to reload your weapons (when there's a big break reload your shotgun, plus you can actually reload during the breaks between each wave) and sometimes it's better to run your revolver out of ammo so you can reload. If you don't know, switch between the revolver and shotgun with your space bar.

I have wasted untold hours on this....

Oh... and shoot after a gun is out of ammo and you will reload.

Finally... the thing that got me at first is you don't use your mouse cursor as the aiming point but just point at the target. It took me a while to get that...

Then again now that I think of it I'm a dumbass. Point the cursor at your target. :|

152 Zombies. How do you switch to the shotgun, anyway?

Shooting them in the pelvis seemed to work pretty well.