Ugh. I finally beat it. I have ZERO desire to play it on elite. Basically, this is what I did:
-Learned the pattern. Fortunately, the game does not seem to arbitrarily choose which side certain enemies run up, so I knew when they were coming and could kill them. When the shock troopers with the sub-machine guns came in, I was waiting with the grenade launcher.
-Suppressing fire. Instead of trying shoot the rocket guys with the sniper rifle, I hung out near one stairway and popped out to take a few potshots with the Hig assault rifle. They'd be constantly running around to avoid my fire, so they rarely got a rocket off, at which point I would jump off the balcony and sprint to the other side. Rinse and repeat for some absurd length of time, and eventually the Intruder crashed through the window (which triggers a save point).
-Ugh, Radec was awful. On hard, he teleports constantly and spams grenades like a n00b with unlimited ammo. It's the cheapest of cheap boss fights. My winning tactic: Hide in an upstairs corner with a flame thrower, set him on fire (which throws off his game completely...and makes him easy to spot) and start nailing him with the grenade launcher. Every few rounds or so, give him another burst of fire to keep him from going out, which stops him from 'nade-spamming.