Global Defense: Zombie War Strategy is actually pretty simple. Keep all enemies moving as slow as possible with Frost and Shockwave Turrets then hit them with the strongest weapon you got, Tesla Turrets. Using Radar can be helpful though it can take away from building up attack power on other more important weapons leaving you weaker in the long run. You might think that using all turrets for their desired purpose would be the best tactic, but it’s not because you’ll be spending cash during waves to upgrade less useful turrets giving you less to spend on the big hitters which leaves you weaker. You can try it yourself by doing what you might think works best, then use the weapons listed above and you’ll notice a night and day difference.
More info at the Global Defense: Zombie War Guide Wiki
You biggest obstacle is having extra points to upgrade in the Armory enough to beat the current level. At first levels may seem easy, but they will get hard fast and you’ll find yourself grinding for points. The best way to get points is to invest in the Rewards section of the Armory where is says “GET MORE UPGRADE PTS(%)” which is maxed out at upgrade level 30. This will increase the amount of points you get after completing a level and should be the first thing you invest in and max out. The absolute best place to farm for points is on Level 3 Saudi Arabia – Hell. It’s 20 waves, can be completed in just over a minute, and will earn you 3251 points with a maxed out upgrade. You can get over 100,000 pts an hour if you stay on it.
After a your Tesla is built up a bit you only have to place one in the middle of the field then speed up to x3. I was able to do this while watching TV, movies, and on road trips. Before you know it you’ll have a stack of pts to build up your strength. If you continuously grind Level 3 on Hell you will always need to play it 10 times in order to increase PTS(%) by 1, as the cost increases your earning will also. So you’ll need to play it around 300 time to max out the skill.
Radius is usually cheaper than power when increasing stats and plays a very important roll in taking down flying enemies that can appear anywhere. It also helps keep your units away from the side of enemy paths where exploding zombies can damage or destroy turrets.