After Lexine mentions her headache and Gabe comments on the locked doors, you'll arrive at a branching path. Your choice only determines what kind of enemies you will face in the circular room and the area immediately beyond. I choose the left path because I find it more interesting and because I think the spawned enemy types are a bit easier to deal with than the ones that spawn when choosing the other route.
Beware the Exploders. I got lucky with the one that detonated in the circular room, but usually the damage they do on the Impossible difficulty mode is nothing to scoff at.
In this Chapter, you'll be introduced to Lurkers. Many enemy types from Dead Space 1 return in Extraction, but Lurkers are one of two that don't function quite the same. In Dead Space: Extraction, Lurkers no longer fling sharp barbs straight at you. Instead, their barbs travel slower, have an explosive effect and can be grabbed (and launched back) with Kinesis.
This video ends shortly after Gabe tells the group to "man up" after being separated from Lexine.
This video begins shortly after Gabe tells the group to "man up" after being separated from Lexine.
The second Line Gun Weapon Upgrade is found behind a ceiling vent that is shown when Weller talks about his hallucination of hearing someone crying. (0:13)
The fourth and final P-Sec Pistol Weapon Upgrade is found behind the ceiling vent above the decapitated body on the railing. (0:32)
The first Force Gun Weapon Upgrade is found behind the ceiling vent above the Lurker that Nate hallucinates. (5:23)
Shortly after being attacked by a Grabber and cutting through a meaty wall, Nate will look down at the sewer fluid before looking up at a vent above a platform with nothing on it. (6:58) Behind the vent is the third Flamethrower Weapon Upgrade. This whole sequence occurs just before Weller says: "Lexine was right. Something's watching us."
This video ends just before the boss fight at the end of this Chapter.
This video begins just before the boss fight at the end of this Chapter.
The Chapter 6 boss battle of Dead Space: Extraction is not that different from the Chapter 6 boss battle of Dead Space 1 (tentacles, explosive pods, explosive canisters...), and I think both are the best boss battles of their respective games.
As you can clearly see from the video, you don't want to miss shots if you can help it. On the Impossible difficulty mode, each tentacle swipe costs you roughly half of one health segment. It might not seem like a lot at first, but it adds up very quickly. Fortunately, there are many storage containers in the boss battle room. If and when you can, break them open to resupply ammunition and health.
When you are on the rightmost platform and get a Free-Look opportunity, you can look down at the platform and find the second Arc Welder Weapon Upgrade. (2:14)
If you have a Weapon with a high rate-of-fire, such as the the P-Sec Pistol, Pulse Rifle or Arc Welder, you can simply fire directly upon the vulnerable "eye" during the last stage of the boss battle. (I suppose you could also use the Flamethrower's alternative fire, if you have it.) If you don't have one of those Weapons, you're better off trying to hit the weakpoint with the explosive pods the boss launches.
Beware the explosive pods, especially when the boss starts launching them out in sets of three. They are easy enough to catch or destroy when launched one at a time, but it's very easy to miss some when they are launched in groups. The damage of individual explosive pods isn't that much, but they can add up and take their toll. As seen in the video, I came very close to dying towards the end of the battle simply because I was having trouble dealing with the pods.