The final mission of the singleplayer campaign is going to be very simple. You won't have to worry about anything in particular. Start off by moving forward. You should be able to locate a locked door in just a few seconds (#1). Pick the lock and enter a new corridor. Keep moving forward. There's a single opponent in a nearby room (#2). You should be able to eliminate him quietly. He wasn't expecting your team to show up here.
You don't have to visit all of the surrounding rooms, because you won't find anything interesting there. Head on to the LEFT room (#1). Be careful, though. You will be encountering additional enemy units along the way. Most of the terrorists will be standing near a door that's leading to a nearby room (#2). Remember to aim at their heads, however you don't have to save up your ammunition anymore.
Lean out carefully. Some of the terrorists will be hiding by the destroyed wall (#1). Just as before, it would be best to aim at their heads. Be careful, though. Some of the enemy soldiers may surprise you from the left flank. You have to be ready for these types of attacks. Head on to the next corridor. One of the terrorists will be hiding here (#2). You may try aiming at his hand/leg or send him a grenade or two.
Keep pushing forward. You will be encountering new units quite regularly (#1). Thankfully, the terrorists will be operating in very small groups, so you shouldn't have any problems eliminating them. You will reach a new location in just a few seconds (#2). Stop here and get ready! This is the final area of this mission. Make sure you've reloaded your current weapon!!
Despite what you're probably thinking, you won't have to do anything out of the ordinary. I would recommend that you pick your targets wisely. Remember that you're not rushed by anything. The main group of enemy soldiers is going to be located on your left (#1). Start shooting at the terrorists. Remember that it would be best to aim at their heads. You might also consider using all of the remaining grenades. You won't need them in the future. Once you're done with the first group of terrorists, concentrate all of your efforts on the soldiers standing by the landing pad (#2).
Start moving towards the nearby stairs (#1). You have to be very careful, especially since there might still be some terrorists in the area. Get down to the lower level. TURN RIGHT. There's probably a single opponent hiding here (#2). You have to get rid of him quickly.
Start moving forward. Be sure to stick close to the LEFT wall (#1), so you won't get lost. You shouldn't encounter any new units at the moments. Order your team to STOP, once you've reached the stairs (#2). Make sure you've reloaded your weapon!
Use the stairs to get to the landing pad. TURN RIGHT. You should see at least two additional units. Take care of the left soldier first. The second terrorist will be Vanderwaal (#1). You might want to consider aiming at his head, however this is not necessary.
CONGRATULATIONS! You've just finished the game!!!