Its pretty simple. From the room you start off in, go up the stairs and to the right. The room with the hole in the middle of the floor is pretty much the only place you stop. Then get all the doors in the map open, and just make the loop. When you come out under the stairs in the main room, you can throw a grenade down the stairs to the theater to get a crawler. It also helps in the later levels to turn on the barrier right there. Then you go to the room I mentioned earlier, stop there, have someone rebuild the window, and hold off for a little bot there, but not too long, cause they will start coming up the stairs to your right. We like to keep someone with a regular AK-74u that you can get outside since its a good money maker, but in the later levels, just know that it does almost no damage. It is just there to make money to keep the gates on.

We got lucky and both got ray guns and monkeys really early. Then my buddy got a thunder gun and we upgraded the thunder gun and ray guns. The upgraded crossbow is also nice if you have more than 2 people.