Cosmocommandos are coming to that alien planet. This is a fast extraction like roguelike. 

Shoot aliens, shoot lots of aliens. Around 50 different species of aliens are distributed in the planet. There should be at least 3 different alien types, each of which have own boss. Typically, there are around 50 enemies around, so prepare for shooting, for lots of shooting. 

Collect resources and return to base for upgrades. If you get killed in action, upgrades are lost but not your stored resources and base research. This helps preparing for attacking. Also, co-op fully available with friends. DLC available that speed up upgrade process, but base game should be still be completely available for non-DLC owners.