You only need 3 guns! PMC, Scramer and Boneduster.
Run into the first room with a charged Screamer, shoot the ceiling. Turn to your right, and pull the switch. Then use you thumper on the burning enemies. Load up a Bonduster charge shot and run to the end door before it opens. As it opens blast the fuckers!
Pick up the mini gun and munch everything in site and same into the next room. If you run out by the time you make it into the ground floor of the next room, just use the Boneduster for Legless/Topless/Torpedo kills etc. Remember to not let go of RT unless you need to cool down!
On the next part load up a Screamer charge shot again. Let the enemies flood in, shoot ceiling and thumper. Grab a mini gun and take out a fair few of them. Maybe best to use you last Screamer charge shot then use the mini gun. Extra points for afterburner etc.
Door you originally walked through will open soon, so charge up another Boneduster and as it opens blast them all. Maybe try a Torpedo added in to it for extra point.
Finish off anyone else and done! If not, do the same again and you get get some extra lucky points! This is actually the best and easiest one to do along with the first lot!