Follow Cruz and O'Neal. Be prepared to face Xenomorphs almost immediately. Look for Spitters first, avoid their ranged attacks and keep moving all the time. Make sure to use the motion tracker; a lot of the aliens will be climbing walls or hiding on rooftops.
Move ahead, one step at a time, and keep checking your back; a group of Xenos will show up behind your backs, near the place where the mission started (the above screenshot).
As soon as you get to the closed door leading inside the colony's building, run to the disabled sentry turret on your right. Face it in the direction you've just came from.
Get ready for some Xeno extermination and make sure not to let them too close to where attempts at opening the door are being made. Don't forget about Spitters, there are still a lot to detect and eliminate.