I recently read this on the Marathon Spoiler Guide:
"Alternatively, instead of first letting the tanks and Pfhor kill each other, you can make a run for the door, charge up, collect ammo, and save, then you can hit the switch, leave the small room and ride the elevator (still avoiding the tanks), enter the cross room to activate the S'pht to follow you back into the tank guy room. You should wait in the side room, poking your head out only once in a while once a battle ensues. Done properly, the tanks and the S'pht will fight each other, leaving you to clean up the weaken remaining enemies."
I decided to test out this technique on Normal, and thought it would make for a great demonstration vid. I promptly hopped in on Major Damage and did a nice demo, which actually turned out quite good - it even demonstrates some very weird monster behavior on platforms. Without further ado, here it is:
A few things to note:
1) Going up and down the elevator is tricky, and I couldn't help but get dinged up a bit. Nevertheless, that's on Major Damage! All in all, you shouldn't let that little slip-up discourage you from using this trick: it's a very easy thing to pull off, once you try it one or two times.
2) It's good to try and get every enemy involved: that's why I shot those distant Compilers when I first went up - I wanted to make sure they'd join in. Unfortunately, two Fighters didn't join (or at least one of them ran off, and the other one just sat on his ledge). You can see me shooting one of them off and into the lava with a single pistol bullet (which is quite hilarious).
EDIT: There always seems to be a Fighter stuck on that ledge, and it always seems to take only one bullet to knock him into the lava. It's not very often you get repeated hilarity in Marathon!
3) As with most films, I do a bit of a dance at the end. I started doing one earlier, but noticed a red dot on my radar, and quickly stopped to find the alien. Soon enough I found the other blue Fighter, who did a spectacular attack while on the elevator. Would've been fun if I had intentionally stood in front of that wave of projectiles! Basically, when you come to my first little dance, don't stop; the film continues a little more.
Enjoy, and feel free to critique or provide useful tips!