This is one of the latest missions in the game, just play to unlock it.
*** Spoiler - click to reveal ***In this mission, the trickiest part (for me) was the crowded escort mission where in a rift you and 2 AI teammates need to defend the key. There are a lot of enemies coming from different angles. Make sure to take down any flying vehicle, then prioritise tanks. the small adds are nothing more than an annoying distraction. When you're running out of life just jump and glide around and freeze a good chunk of adds to kill them quickly regaining health. Carefully clear the room and you should complete the mission without too much trouble.