The best strategy to beat this game...
Here's how I managed to beat this game:
First, don't sell any of the units you start with (or any of the ones you'll buy for that matter). As soon as possible, buy regular gunners, until you get 7 of them. Then, buy three bazooka troopers. And finally, buy repair units, up to 8 of them. Now, your 18-men squad is complete. Do whatever you can not to lose one of them, because you'll be needing high-level troops during the last levels.
Now, start saving up, and buy a mortar. It will start getting useful pretty soon. After that, get a machine gun turret. The air-enemies-only levels are coming, and this will come in handy, in order to keep your men alive. Buy a second mortar whenever you can, then a second machine gun turret. Now, you're ready to go! You can use the rest of your money to repair your walls between levels, but your eight high-level repair units should be more than enough.
And remember, if you ever lose one man or one turret, it's better to just quit the game and start at the beginning of that level. Once again, you WILL need high-level units by the end of the game.
That's how I could finish this game after many tries and strategies. And with this one, I finished it without ever losing one man or one turret, and with well over 6000 bucks for the last level. So, feel free to try. ;-)