I have played through easy, medium and Hard. Working my way through psycho. What a pain. The trick is to start small and recycle. I use the very last level on Hard to level up my Commander. It is Tedious but it works. The trick for the bosses, ANY of the bosses is to slow them down! In the beginning, find a good choke point where you can blast away with Bombs. You will definitely want to get the three repair drones out there so you can get some cash flowing. Do this once you feel like you are keeping the little buggers at bay for a few waves. Then, once you have a decent equilibrium or… balance of power, start building sniper turrets. Upgrade them and boost them with power boosters. a fully upgraded power booster will give you an additional 300 of damage to a fully upgraded sniper turret. (See my attached image) Like the cat said earlier, get your slow towers for both air and ground out there. The winged flyers are easily taken out with fully upgraded missile batteries.
The only weapons you will need in this game are laser, bomb, slow (Both air and ground), sniper, booster, and missile. Anything else is a waste of time and money.
Hope this helps.