Go back to the ground floor, but make sure to replenish your supply if you can. Before leaving the vehicle bay, search the main area for the entrance to the underground tunnels, pictured on screenshot 1. Check them out to find the Dog Tag from screenshot 2 (LT P. Watkins).
Make your way towards the newly unlocked exit of the vehicle bay and you'll return to the surface. Move ahead, but stop where the screen points; you're about to face an alien Crusher for the first time.
Focus primarily on dodging to avoid the alien's charges. Don't expect to find cover behind the barricades on the left, though; the Xeno may easily destroy them.
Each time you manage to perform a dodge (otherwise you'll be crushed into the ground and lose some health), hit the Crusher with what best weapon you have. My recommendation is the tactical shotgun, but you may obviously use a rifle or the grenade launcher as well. Soon, other Xenos will join in on the fight, but take an interest in them only if they get too close and may pose a direct threat.
Keep dodging and firing until you've done the job. Get rid of the other Xenos and follow your teammates towards the landing pad. Wait until Cruz opens the door to the new building.
Go inside and start exploring to stumble upon the above Dog Tag (PFC D. Rechner). Move forward and jump down to get to the lower level. Exit the building and approach the landing pad to receive new orders.