First time posting something What you want to do is spend rounds 1-3 in the first room. Buy the rifle that kills in one hit but don't use it. Rounds 1 and 2 you should just move from barrier to barrier checking to see if a zombie is there, if there is knife them while hitting the repair button to kill them and get points for repairing. If a zombie does get in just knife them and move on. At wave three you have to shoot them, then stab them. If you do this while continuing to repair you will have 3000. Then clear the stairway IGNORE the help room. Run and buy the sawed off shotgun and run back to the stairs. At the top of the staircase is a wall and a window. Push yourself against the wall so zombies can't get you from behind. Then blow everything away until you have 2000. Near the trenchgun is a cabinet. Open it to get the flamethrower. Now that you have that run to the big white square outline (I think its grenades I've never used it) Anyways its by the stairs and incredibly hard to miss. Put your back to the wall and burn everything to death. With this zombies can only come from one direction and It is a lot easier
Made it as far as round 14 on my own. By that point, everything's nuts and you have to constantly keep moving. The flamethrower can be quite handy, but the Trench and the BAR have been the most trusty arsenal for me so far.
That'll work in the beginning, but you NEVER want to put yourself in a corner, you won't have anywhere to go if (or rather, when) a zombie gets too close; around level 9-10 go downstairs and run through and around thr tons of zombies that will be chasing after you. Also, what rounf has everyone gotten too by themselves; I've gotten to round 14.
It's upstairs by the cabinet where you get the flamethrower. I recommend the Double Barrel, but that's just me.
