Well
The main unit that you can use is the fully-upgraded Tack Shooter which you put inside a corner (preferably inside a hairpin turn). Also you need a fully-upgraded Boomerang later on in order to destroy the lead balloons, and finally you can switch to super monkeys once you can afford them. I tried using dart monkeys, cannons, spike-o-pults, and ice balls, but they just weren't cost effective compared to the Tack Shooters. The Monkey Beacon can help you if you have a lot of tack shooters in a small area, but not until late game. Also, if anybody is having any trouble in the late game (levels 40-50 or so): just use road spikes as a last resort to stop any balloons from getting past. By the time you get to level 40-50 you should have no trouble affording the road spikes.
There is much to be desired as far as replayability in this one. You've done a great job at improving these as you've made one after another.