The first act of the game has four missions. These missions are extremely easy, since it is the begining of the game. The best strategy is to spam out Scorpions for the first two missions. After that, spam Stealth Tanks with their Rocket Upgrade. They can destroy ANYTHING. Once you get the Black Hand of Nod, build Purifiers and upgrade their flame throwers with the Secret Shrine. Take the Purifiers and just order them to pass by the enemy units and buildings and enjoy the show. Also, remember to set units to Aggressive Stance.