Or I too dumb to convey it :) basically you click your units to select them and click the aliens to make the units shoot them. If you can shoot and kill all aliens in the 4 waves, you win. To get some more firepower, you can select a unit and click a power mode button, but this will also make them unable to move, so you will have to be smart about it. Hope this helps, and if you have any questions, shoot!