For people having trouble, all I can say is: lasers. Buy the $100 turrets to fill all gun slots (don't even bother upgrading), buy elevator shafts as soon as they are available, and replace turret guns with lasers every time you get 4k to spend. Once you fill every slot with lasers, start upgrading them. Lots of different gun combinations work, but none so well as all lasers. :)
Not a bad game for a little diversion. Good concept, and fun to play. Yes, it's too easy, and there are some balance issues, but it's a flash game, not an MMO, and fun comes in first in my book. :) 4/5
 "The Next Floor")
 "Kingdom of The Wind")