1. Stay in the bottom left room
(guard the 4 barricades just let the other ones alone,
it's too hard running around everywhere).
2. save up for the MP5N submachine gun,
(don't buy any other weapon).
3. keep buying sentry guns
(it doesn't matter which one, you gain money
from them any how).
4. keep buying sentry guns until the MAG machine gun
(VERY VERY STRONG).
5. buy sentry guns and upgrade barricades as much as possible
(you should survive until then).
if you see a white dot on your radar, do not let it reach the barricade