I was playing this game with not much strategy but around mission 10 it suddenly became really difficult. This is where I actually had to start thinking. So I figured I give a run down of some hints that will make this game a lot easier.

1) Unit order cycling- You can freely and instantly reorder units on the fly. When coupled with a shield unit, you can switch the 1st and 3rd position so as to put a fully shielded unit in the front and the weakened unit in the back (and allow for recharge). This is probably the biggest advantage tactic available.

2) Collect commander abilities- try to destroy as many enemy units as possible as this will give you some money, but more importantly give you more special abilities to use during the combat.

3) Unit choice- In later missions I find myself often going with a 3 unit setup. Missile launcher/Shield/Missile Launcher. I put money into upgrading the missile launcher units as they can destroy weak towers with one shot. It also makes unit cycling and repairing easier as the caravan is not so long and your units will be in range of enemies for less time.

4) Commander as decoy- Before your units reach the enemy, put your commander on the other side of towers, this will cause them to turn and face you and give your units an extra second or two of protection.

Hope this helps.