When you enter a Mission, you’ll face a series of waves of enemies. Your goal is to defeat all the waves. At the end of the battle, the output will be:

Victory - you win when you defeat all rival XEN Units. At least one of your XEN Units must survive. In this case, you will receive a Chest reward.

Defeat - you lose when all of your XEN Units have been defeated and at least one enemy Unit remains alive.