Upgrade your machine gun as soon as you can, since you'll use it throughout the game. Whenever you see an enemy unit with an exclamation mark above its head, make sure you gun them down immediately or they'll cause a great deal of damage to you. Tap the green rocks on the field to get some money. Because the respawn time increases the more you die, it might be a good idea to switch generations after about three deaths otherwise you'll have to wait for too long. The best way to get through some of the earlier levels in the game is with the help of support units. Hire as many units as you can to protect your back and help you through levels. A sniper rifle is pretty much essential after wave four because of some fairly vicious enemies like rhinoceros beetles that you need to snipe out as fast as you can.