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.

Basically, yes to all of the above. When your launchers and shield are fully upgraded, maybe look towards a Tank or a Dragon and a secondary Shield unit behind it. If you can get to that point, anyway.

The commander's abilities are extremely useful. All of them. Good shield use can render the repair irrelevant, but the airstrikes, decoys and smoke bombs are all very handy.
An airstrike can take out the energy drainers even as they're draining the power from it. I never face off against those things if I can help it - a quick strike and they're done.

Putting your vehicles on a short, safe loop is a great way to give the Commander time to rush out and murder things with airstrikes, by the way.

Keep doing the unit order cycling, that is the most important strategy. Also take your time, sometimes doing a few circles around a cleared out block with your units to get a couple of easy shots on an enemy which is just barely in range from one of the corners helps a lot. If you kill a lot of units (even if you don't have to), you get power-ups that really help.

This is the year 2024 chiming in.

I have all the money but can only ever have 6 units. That is lame AF. It's no wonder it's taken me over a decade to attempt to finish this game. I might just skip the final level (which I'm on).