First, I stay in the first area until I have $100, then I go into the street and buy the Mp5. After that, I just go around, killing zombies until I get $151 and open up the Reseptionist's area and get the shotgun. Buy the first upgrade for fortifications now. Then I save up and open the Latrines. Buy the second fortification upgrade now. After that, I save up and open up the Car Lot, buy the third fortification upgrade, and open up the Workplace and pick up M60 and Sentry Guns. Buy forth fortification upgrade now. I Then go back to the Latrines and fight around there until I have enough to buy the G3. After that, I go up to the Mess Hall, buy the fifth and final fortification upgrade, and fight there until I have enough to enter the Infermory and get the LMG. Once you have done all of this, I go into the latrines and stand on the far left of the room. I place a $10,000 sentry gun so it can cover anyone who comes near me, but not the entrance to the latrines, and also cover the left entrance to the latrines. I then stand on the right side and just shoot the heck out of the zombies. By this time, it should be about wave 27. Whenever you have the money, place another $10,000 sentry gun in with the other one, and another one on the other side of the wall in the latrines. Just continue to place sentry guns with your extra money, and eventually you will win.