For people who think this game is hard...
It's not, the trick is to buy pretty much only 1 type of unit, just enough to where you're sure you can beat the next level, and then use the leftovers to buy as many upgrades as possible. Then when you're attacking deploy all your units in one large wave. The game may slow down for a moment (especially when you've got 300 infantry out there), but it will speed back up as they die.
To the creators: try to make it so that's there's a strategic advantage to different units at different times, currently there's no reason to diversify one's army.