Sometimes you can't save the entire planet, however you can help it if you figure out how to guard a significant laboratory where scientists are searching for a remedy to the present issue. It's not on your interest the way to manage an alien invasion, and you wouldn't know it anyhow. So that you do what you understand the finest and it's controlling your firearms. Monsters understand they need to destroy the laboratory, but they won't pass . Or do they? Shield the zone and don't allow the monsters kill you. Don't cross the red line at any price, since you get to a dangerous zone and you instantly perish. Have fun.