(1) You start the fist level of this third and final part of the game in a room with 3 passages leading from it. First shoot the 6 alien canisters--each yields a single derm patch.
(2) Now you want to take the left corridor first. Keep a sharp eye on your motion tracker as you approach the apparent end: it is an invisible wall, the first of many, that you (and the aliens) can simply walk through. Lure the 3 adult dog aliens behind the wall out 1 at a time. Since there are a lot of shotshells available in the immediate area, stick with this weapon. Seven direct hits will do the trick. Now enter the room and grab the 2 shotshells; although you probably have no use for them, there also happen to be a pair of clips lying here too. Blow open the 4 alien canisters for derm patches.
(3) Backtrack to the starting room and, if you need them, grab the 2 shotshells in the middle corridor but go no farther. Now take the right corridor. In this long "tunnel" you will first encounter a trio of adult dogs and then a pair. Along this route are also a shotshell and a med kit. Proceed past the first junction, marked by a curtainlike veil and a clip, and move toward the end of this tunnel. At the wall you will come to is a lift with a med kit on it. DO NOT go down the lift or the consequences will be dire! Instead, tiptoe up to the edge of the lift to activate it and make it go down, WITHOUT you on it. This takes some practice, but will save you from having to do a lot of backtracking and a lot of re-killing of enemies later.
(4) Now return to the unexplored junction you left behind earlier and enter it. Shoot the alien handler that attacks from your left and proceed right. Grab the shotgun shell; there is a derm patch lying next to it. This derm patch, along with the next 4 you'll find, are special 10-up patches that will give you a 50-point total health boost! You will meet up with a single alien handler and then kill a pair at another junction on your right, but continue to your left momentarily. There is 1 more alien handler to kill and, if you didn't grab the 2 shotshells in the middle corridor of the starting room before, you have the opportunity to do so now.
(5) Enter the junction you just passed up and stay right. You will find a derm patch near one corner and in the next lie another patch and a clip.
(6) Stay right at the junction you will come to and kill the alien handler in front of you. To your right, halfway up this segment of the corridor, is a room hidden behind another invisible wall. Enter to grab the body armor and activate the alien switch within. Exit and continue; another handler waits in front of a lift just ahead. Kill him and then make a full circuit of this corridor to grab a single derm patch.
(7) Back at the lift you will find another derm patch. The lift will drop when you step onto it. Stay to the back of the lift and only the adult dog alien directly ahead of you attacks. Pulse rifle or shotgun him and then take care of the dog on the right. There are 2 derm patches here as well.
(8) Head to the right until you come to a secret room behind a pulse rifle magazine and an invisible wall. Dispatch the adult dog that emerges and activate the switch; there is also a med kit inside.
(9) DO NOT proceed forward--instead, go back to the lift you came down on and now proceed to the left. You will find another derm patch and, beyond it, another adult dog. As you continue, another dog exits a secret room on your left and attacks. Kill him and then grab the night vision goggles, but IGNORE the switch inside. Grab the derm patch in the corner ahead and deal with another dog in this area.
(10) Now select the smartgun and proceed forward cautiously. You will pass a lift on your left; what you want to do is dash forward to a room with 5 adult dogs inside behind a transparent veil. The trick is to go far enough forward so that all the aliens come after you and into the effective path of the smartgun, but not so near that the closer dogs can flank you. It will take some practice to learn how broad an angle this weapon can effectively cover. Once there are only 1 or 2 dogs left running around, you can save some smartgun ammo by switching to the pulse rifle or shotgun to finish mopping up. You can now enter the room for a look around; at the far wall is the lift you dropped earlier with the med kit on it. This is where you would have ended up (with a lot less room to maneuver) if you had ridden the lift down earlier rather than merely activating it. Ignore the lift and continue out this room and left. There is a single derm patch down here you don't want to miss.
(11) Now reverse your route and step onto the lift you passed earlier (NOT the lift in the room with the 5 dogs). You will drop down into the last corridor of this level. Stay to your left. Grab the shotshell, kill the adult dog alien, grab the pulse rifle magazine if you need it now. If not, you'll be coming back this way in a moment and you will want to save it until then. Up ahead is a pair of adult dogs guarding a single shotshell. Beyond them are a single dog, a flame thrower, and 1 more shotgun shell. As soon as you touch that shotshell, another adult dog comes after you. Lead him back to the last dog you killed and dispatch him there so he is not a problem in the near future. At this point, reverse your route and backtrack all the way to the lift. The reason? You are approaching the lift to the exit room. Walk on it and down you go, with no chance to finish exploring the rest of this corridor or killing the remaining enemies. You may have to kill some revived enemies along the way. At the lift, this time stay to the right. Before reaching the exit lift, you will find a shotshell, shotgun, and 3 adult dogs.
(12) You are now ready to exit. Locate the lift by the lines on the floor of the corridor and walk onto it. This lift is almost "fussy" in the sense that you must step on just the right spot to activate it. Keep walking around on it until it drops. At the bottom is another special 10-up derm patch and, just behind it, a wall that disappears to reveal the exit when you approach it. One level closer to the final queen!