Make sure you save at the very beginning of the level. 'Nuf said.
As soon as you teleport in, you'll be surrounded by Troopers and exploding Bobs. There are a few strategies for this level which are outlined below....
-The RUN LIKE HECK way-: As soon as you see it, hit the switch that is by the door, and start picking up ammo (avoiding the shield powerups, you'll want to save these for last). Kill the buggers that approach you from either direction while you wait for the airlock door to open. Once you can get through, grab the purple powerup, plow through the door, and head towards the left, avoiding a mass of exploding BOBs and heading for the save terminal. Save your game, then return to the main level by turning right from the term, and following the left wall. You'll approach another airlock door, which you'll want to hit the switch for immediately. Pick up the ammo that appears in front of the door, and then run against the left wall as before...you'll eventually come across a purple powerup; grab it, then return to the airlock door, which should be nearly open by now. Run through, and you'll be able to calm down for a bit. You'll enter an area where Hunters will appear...kill these, and you'll soon find a save term. Use this. Then, once you've read the term, run forward on the ledge, past anything that teleports in. Take the right turn, and run up the stairs, to the left, and out onto the ledge. Grab the rocket launcher (finally!) as well as ammo and the shield power up, then retreat back into the hallway. Turn left, and hit the last airlock switch. At this point, you can defend yourself safely here from any attacks until the door opens.
A slight variation on this method takes the left branch after the Tycho term, which leads to a dead end ledge in the rocket launcher room. Run to the end of the ledge, spend a second or two killing from afar, then run back and follow the rest of the above to get to the launcher.
-The I'M BORED LET ME WASTE MY AMMO method-: Immediately back into the corner of the wall and the door when you teleport into the main part of the level. Two or 3 aliens will approach you, but you can deal with these easily. From this point, you can see the troopers on the ledge, who'll all fire grenades at you that fall short, but they will never jump off the ledge. You can pick these off one by one with that extra magnum ammo you've got. Eventually, you'll clear the top ledge, which will remove most of the Troopers from the level. You can then proceed through the level relatively easily without problems except from the Hunters (but these should be easily to deal with). Note that this first part could take nearly 30 minutes to finish this way.
-The LAZY CHICHENSH*T'S method-: Proceed through the inner airlock as above, and save at the save term. Spend a bit of time clearing the ledge that was to the right of the airlock door (as you were coming through the door). Once it is momentarily clear, grenade jump on top of it, then take the first left you come to. You'll run across a few Troopers which you should kill, and then you'll find yourself overlooking the room with the rocket launcher in the center. Troopers will teleport in below, but they will begin fighting each other with missed shots. Once the way below is clear, jump down to the right, which gets you to the stairs, and a direct path to the rocket launcher. Note that this method avoids the second airlock as with several other rooms, but you need to be at a good health level to do it.
Alternatively, to the above, get inside the first airlock door, but DON'T save. Instead, move underneath the ledge on the right, and wait for a few of the BOBs to approach you. One or two should eventually get close, and let them explode. You'll end up on this ledge, which will make clearing out the rest of the level quite easy. However, there will probably be a green trooper that will be ready to take you out. You should immediately turn around, and using a weapon *other* than a shotgun, take him out. The recoil from the shotgun may knock you off the ledge. At this point, you can run and dodge the aliens until you get to the forementioned 2x generator. These methods are good if you are on a relatively slow (030, 040) computer.
You can also get to this ledge by using the rising airlock door as an elevator, assuming that you are fast enough to clean the area for a sufficient length of time and that you can make it onto the rising door. This then will leave you at full health once you are there.
When you are done with clearing out the level, you can take the upper ledges back to the earlier parts of the level to get more ammo and shields.
If you jump forward off the platform with the rocket launcher, and into the plasma, there will be a corridor that leads to one of the ambush ledges earlier in the level.