This one is pretty easy to do on the Dark Star level
When Miller and Vlad go to fix the door Mutants will jump on your rail train one on the right back window and one on the left.
Use your flame grenades and hit them while they are on the window and it will create a firewall that will kill at least 3-4 more per side.
when you run out of flame grenades or you see the anomaly come down the track, reload last check point and repeat!
Good luck, hope this helps