Keep close to your teammates and follow them on the way to the checkpoint. After arriving, kill the guard at the tower first and your teammates will do the rest. Next, head to the checkpoint and hide the bodies. Wait for the enemy convoy to arrive, approach the driver and stab him in the neck when prompted. After taking out the driver and the passenger, hop to the jeep and stay put.
As soon as you exit the tunnel, the countdown for the blackout will appear. Wait until you reach the base then disembark from the vehicle. Follow your teammates inside and wait until the blackout is triggered. Equip your Night Vision googles by pressing the UP Dpad. Go weapons-free and kill all the guards that you encounter. Continue forth until you reach the rack of servers which your teammates will check. There's also a Rorke File nearby.
After your teammates successfully patches in the enemy system, follow them and take out the guards you'll encounter by the door. Continue forth and your team will stop by a large door. Pick up the drill and start drilling on the marked spot on the wall. After drilling a bit, stop and wait for your teammate to mark another spot where you can drill. After that wait for them to plant the charges and detonate the thermites to take out the blast door.
Head to the next room and start shooting the guards on sight. More enemies will appear from the left so take cover and take them out as well. Continue to the corridor to the left and encounter more enemies in the next junction. A well-placed grenade should be able to take flush them out. After clearing, continue forth to the next exit with your team. You'll be in the catwalk outside. Follow Keegan and watch him throw off a guard you'll encounter. Continue until you reach the next door. Enter the base again but hold your position and let the converging enemy troops pass by.
Follow Keegan outside then wait for him to take out the two guards. Continue moving forth and fight your way through the rotunda then continue to the next room. Continue to the control platform ahead and drop your bag in the designated position on the table. You have to set up your defenses and hold your position while downloading the data from the base's mainframe. While your team is going at it, pick up the teargas, place the shockwave and thermobaric mines around key positions in your location, and deploy the auto-turret near the walkways or staircases near your location.
Defend the area until you successfully downloaded the file. Exit through the door at the back then ride the elevator up. Once you're back in the main floor, blend in the enemy soldiers and just follow your teammates until you reach the jeep. Stay put and you'll be stopped by some guards near the exit. The exfil countdown will start as well. Your cover will be blown and you'll have to make a hasty exit. Man the mounted turret on the vehicle and use it to cover your escape.
The pursuers will eventually include snowmobiles as well. Once you're running along the river, aim at the ice in front of the vehicle to make the surface collapse and stop your pursuers on track. Keep shooting at them until you reach the extraction point. An allied sub will surface to extract your team. Mission complete.