: Follow the subway tunnel around and take care of the posse of floaters, then
: enter the column room and immediately head to one of the walls. Pick up
: the ammo that teleports in by skirting the edges of the room; this lets
: you avoid a huge teleporting ambush.
Once you leave the small room containing the oxygen recharger/PB and drop down into the subway tunnel, it is possible to get back to that small room. It takes a bit of practice, but while running, you can use the tracks within the tunnel to launch yourself back into the room. So if you want to take on the ambush Mark mentions, then you can return afterward to recharge and save before heading into the final room.
: You
: can get up onto the ledge that circles the room by running over the crates
: from east to west; kill everyone and hit the switch in the west to open a
: small door to the east. Save and read the terminal.
Also if you run across one of the large crates in the middle of the room, it triggers a nice ammo cache on the nearby crate.