The group of Monolith that spawn when you open the coded door.
I tried two different ways, both were very effective.
Before you climb the ladder, look around and collect the explosive barrels, and the gas tank, if you have mines it's even better.
they spawn in close to the ladder, place the barrels and gas tank next to the ladder, place a couple of mines behind the double steel boxes as they'll run behind it.
place the rest of the mines (six would be good) in the small gap between the wall and the steel chest a bit further up from the ladder (choke point)
let them all spawn in before you start shooting at the barrels, take out any left behind from the explosions (I found 1 still alive)
or..........
clear all the wooden crates that are sitting on top of the steel boxes, hit the decoder and get behind the double steel boxes and rocket spam the whole spawn in area, they'll head for cover and stay there as long as you keep spamming, once the door opens back into the room while still firing till you hit the level change.
I've tried both ways and didn't take one hit
edit:
http://busetibi.homestead.com/files/ss_ ... cofag_.jpg
as you can see they try to duck behind those steel boxes once things get hot,they then run into the mines
p00f! all gone