Good game.
Level 2 isn't that hard. What I did was put the turrets right next to each other in a line to make a concentrated shot that can kill the units fast enough, both above and below the base. You'll need to work quickly and replace any turrets that have been taken down, all the while building your power plants at the same time until you can get your factory.
Actually, I managed to beat level 2 without upgrading my turrets, only because I couldn't figure it out until I clicked on the factory and realized I still needed to buy the upgrade.
Now level 3 is a bitch. I'm fine until it gets to the armored boats, and all of a sudden, I can't kill them fast enough even WITH the upgraded turrets. It's all about the placement of the turrets in this game, so I'll still need to figure it out for this level.