Each campaign is going to vary for you based on the randomness of the game itself, but the following are a few suggestions to hopefully help you out.
In the early stages of the game try to kill all zombies before the sun goes down. This will conquer the territory and give you points at the end of each round for the rest of the game. Capturing territories early on is definitely the most important thing.
Early on upgrade the helicopters return time rather than the speed that it picks up civilians. Its already relatively fast (about .24 seconds initially) so you want to make sure your helicopter will drop off and get back as fast as possible to pick up more civilians. Do this for at least the first 3 levels of the helicopter.
If you're finding the stages too difficult use the minus key to slow the game down. This can help in the beginning to give you some thinking time.
For a few of the different units of the game here's a few strategies I like to use.
Start them off covering one side of the zombie entrances, and fall back to your main landing area over time picking off zombies as you go. In my opinion this is the most important unit in the game. They are really good for cleaning up any stragglers. I like to throw a point or two in accuracy first and then work on their speed.
I try to position these units so they can cover one set of the zombie entrances. Then move their target location closer to the helipad as time goes on, you can get pretty good coverage if you place them in one of the corners of the map and target the opposite side.
I put these a few blocks ahead of where the zombies are coming out (2-3), this maximizes amount of zombies that will be killed. I make sure they are all lined up with each other vertically, since you have to set them off all at once.
If your still finding the game too difficult don't be afraid to turn some of the mods on, like
Chooser. Triplets allows you to have more than one of each type of mercenaries, and Chooser allows you to choose which mercenary are on each months lineup.