Tip for surviving most of the game, stick back and pick off the zombies as they come after you, and keep your distance, so they can't hit you, as the game progresses use some of the older guns that you get such as the shotgun, so you can save up on ammo.
For, My heart up your ass, rest your fingers before trying to constantly retry it, because your fingers will start to cramp up, and won't hit the keys..