You can do this solo, or with a group (recommended)
The mode is "Extinction", you and your squad's job is destroy the Alien Hives.
At one point, you will be protecting a Helicopter from "Scorpions" who attack it with projectiles.
The chopper is working at destroying a larger "Hive" that
can't be destroyed by using the drill.
You'll be fighting aliens for a good while, be patient, communicate and supply each other with boosts and ammo.
Once the hive is destroyed, you will unlock "City Dweller" for 20g
Good Luck.
Thanks to Xbox One Game DVR, I can show you how it unlocked for me.