I did the chair thing after the second attempt. It works, I wait until the big fat guy is outside the second door, open the first, wail on him with the chaingun, and watch him fall, but when I eventually step outside I kept dying instantly by guys I couldn't see. Two or three thwaps would kill me before I could even locate a target with the binoculars.
Well, there's more than one rocket guy. There's about 7 or so. Just keep popping out to "wake" them, then run back and let them come to in front of the door and kill them.
Then, when you go back outside, there will be several of the invisible jumper trigen, whom you'll need to spot with the cryvision thingies. Easiest way to do this is to crawl to the edge of the crater, then whip out the goggles. Once you find one, turn off the cryvision and focus on where he's at (you can faintly see their outlines), and nail him with your sniper / rocket whatever you brough.
There's three or so of them, but they're pretty easy to kill.
Another advantage of the chair thing is that you can go back and get some more health from that room (since there's 300 or so).
That oughta do it