lead the brute into another district and kill him before he dissapears if done right the engineer wont be affected(done this at ONI alpha site area)
I led the brute from the D04 Sunken Plaza into D06 three times. I was never able to keep the Engineers alive after the Brute died in an adjacent district.
Video 1 (Mombasa St 007)http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3fileid=102564647My first attempt is a fail, so forward to 2:07. I lead the brute into D06 through D0604-A. He despawns before I kill him. This happens at 3:05. If you pan cam over to the engineers before the District Doors (DDs) close, you will see them go "poof!" precisely at 3:05. They become horseshoe-shaped ghosts that Enigma can use to further his disinformation campaign (please don't!) Also at 3:05 the DDs were shut- not that it matters. Once you see this, you can end the film. The remaining nine minutes is just me getting set up for the next run.
Video 2 (Mombasa St 008)http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3fileid=102564808This time I kill the Brute before he despawns, but I kill him right on the threshold of the DDs. The engineers go bang normally. The DDs were open the entire time. I didn't like this video for three reasons: I forgot to quit and resume so the action doesn't start until 10 minutes in, the DDs remained open, and the Brute died too close to the door. So skip it. I link it only so that the proof is provided.
Video 3 (Mombasa St 009)http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3fileid=102564819So far this is my best attempt. The Brute buys the farm at 1:31. The DDs are closed (although the lock is still in a yellow status.) I get it done on my first try. What happens back over in D04? The usual, precisely at 1:31. Dead engineers.
What is somewhat interesting in Video 1 and Video 3 is how the Engineers' deaths don't climax with a huge explosion (mind your comments, fellas) but instead just vanish into a small blue cloud sometimes without even doing their normal pre-explosion ascent. I'm guessing that this is some manner of game optimization or simply a theater glitch.
In any event, I could not replicate what OG claims to have done in CM:MS:ONI.
P.S. If you care to see any of these videos in the future, please save them to your machine. I'll be taking them down eventually to make more room for other things.