The objective of War Machines is to battle other machines in order to save your city.
Remember which machine type beats which: Earthquake beats Twister, Twister beats Hurricane, and Hurricane beats Earthquake. Think carefully before you use a powerup. After round 3, repair your machine if it needs it. If not, use Ultimate Mode. The types of powerups you can use depend on your machine status. Upgrade your cards whenever possible. You don't have much time to choose cards, so memorize their locations so you can click them quickly. If your opponent chooses a more powerful machine type, use Resist Pushback.