The game is actually 3 games in one. Try to complete them all as quickly as possible and before the time runs out! After a while a hint will pop up and then if you spend way too much time a skip button will pop up.
The Zeta forces are trying to take over our secondary base, we need you to slow down their progress. The more you kill, the more cash you'll get. Get ready, they're coming and shoot as many of them as possible and reach the Zantor Station at the end of the level to proceed further.