It’s an ugly fight to fend off the xenos to get into the next section. Get your back to the door and hold them off.
Once inside, use your motion tracker to find marines to rescue. One is fine, one is in a room you have to cut into, and one is under some rubble.
Once that’s done you’ll set up a new op center and hold off a LOT of xenos. Be sure to deploy the auto-gun for it’s coverage. The guys will yell stuff, but don’t expect their directions to actually point you at where the bad guys are.
In the next section you’ll come to a courtyard area, and you’ll meet a new xeno. He soaks up an astounding amount of damage, so use grenades and fire until they run out, then aim at his legs. Don’t run in straight lines, and remember he’ll only ever run after you, not your teammates.
At the drop ship, you’ll need to disconnect three fuel lines that attach to the ship. Do it as fast as you can and worry about the xenos after. Look for the full armor set, you’ll need it. Then you’ll just have to thin the beasts out before you can get away.