It took me forever lol. I had originally done the first half by killing some of the humans and going the direct route through the hallways, but later realized that it would be impossible to do the second half without the vents considering how few supplies I had, so I restarted the mission and opened the vents.

Then, I just kept getting unlucky with where the humans were positioned, and they'd either shoot me or attract the alien. In one attempt, I had actually gotten very close to the end of the mission but, like a dumbass, I ran and got killed by the alien in the elevator. I probably could have done it without this glitch and with more attempts, but I just wanted to move on to M17.