4 Dog Tags
Kill the xenomorphs along the way and make your way to the entrance. Pick up the auto-turret and deploy it facing the street. Hold your position and wait until the door opens. Head inside then head left. Check the small room to the left to find a full suit of armor and the legendary weapon, Vasquez's Pistol.
Follow the main path and go upstairs. Continue straight ahead where you'll see a marine getting snatched by a xeno on the ceiling. Make your way back to the lab and rescue the downed marine, pinned under a fiery bed. Use your motion tracker to know the locations of the marines you have to rescue.
Head to the next room and find the mission's first dog tag on the table, beside the microscope. Cut the sealed door nearby open to rescue another marine then head to the operations and kill all xenos to save another marine. Once done, your objective will be updated and you'll have to rendezvous with Cruz downstairs.
Head to the vehicle bay and set up a defense perimeter. Deploy the auto-turret near the windows then when you have the chance, find the stairs leading to the maintenance trench. Turn to the second corner to find a dog tag on the floor. Stand your ground in this area and fend off the xeno attack. There's an ammo cache in the small room in the corner so use it to replenish all your weapons' ammo.
Exit the vehicle bay and go past the gate. Here, you'll encounter a new enemy called the Crusher. Don't waste your ammo on its head as it is invulnerable. Wait for it to charge then quickly get out of the way. Shoot its unarmored body to deal damage. Xenos will appear to join the fray as well. Take note that the concrete blocks won't stop the crusher so there's no use hiding behind them. Once you see it running, quickly get out of the way, turn around and fire. This may take several tries, since the pesky xenos will target you as well.
Now there's a cheap and easy way to kill it without getting damaged (and get the achievement “Anytime, Anywhere” as well). As soon as the gate opens, exit a bit but don't get too far ahead. Find the slope in the left corner of the gate, beside the corpse. This is where you'll make the stand. Now go ahead to make the crusher appear. Wait for it to get near then return to the high ground you found earlier and stay there. When done correctly, the crusher will just walk towards you or stay still while you're laying fire on it. Use explosives and grenades to take it out. There are other slopes or elevated spots in the areas but this is nearest one and possibly won't trigger xeno respawns.
Head to the next door and wait for Cruz to open it. Get inside and follow the path until you reach a workbench. You'll find another dog tag there.
Exit the bay and head to the dropship. Check the crate in front of the dropship, beside a payloader to find the mission's last dog tag.
Refill your ammo using the ammo cache nearby. You have to disconnect three fuel lines under the drop ship. As you do that, a continuous stream of xenos will pour in. After disconnecting the last fuel line, kill the last remaining xenos to complete the mission.