The idea here is to place your guns (towers) in such a way to kill all the bugs that come through. Like Lemmings in reverse
I tend to use the Squirt towers and upgrade them making a killing zone around the middle of the board. The Frost towers are good to slow the bugs down, so are good placed near the entrances. But they don't stop the hard big bugs
Fewer towers and more upgrades also seems a good rule of thumb. I notice that you can direct the bugs around so channel them into killing zones.