Great!
I really enjoyed the first version of this game and played it to completion. I'm currently on the 4th mission. Some people seem to be having trouble with the difficulty.
1) Use the double machinegun. In the first mission you can get it for 2k, and in the others 10k, but by that point, you're always starting with enough money to buy it and something else. It makes aiming a LOT easier.
2) The layout I use is the double machine gun, the sniper shot, the drone creater, and the health buffer. I use the double machinegun primarily, obviously, and the sniper when it's available.
3) Upgrades are important to success. Wait to upgrade the double machinegun until you have 10 points and can up the damage. That's a HUGE bonus. I upgrade the sniper shot 5 times and then just put the points into extra damage. Wait to upgrade the drone creater until you can have them attack. Then just put remaining upgrade points into having more per level.
Like I said, I've just started the 4th campaign, but that layout seems to work perfectly for me.