I got mine near the start of the Contagion level.
Sneak thru the air ducts, watch the woman being taken away, sneak further along and then watch the guys getting shot.
Keep going right, drop into the room. Take out the two executioners. Take the incendiary grenades from the executioner.
Now make your way to the railway. There's a checkpoint right here.
You'll see a red-lit wagon in front of you with a guard. To the left, and slightly up some steps are three guys talking. Let loose with all your incendiary grenades. If the cheevo pops, fantastic. Reload the checkpoint and you're back on the railroad.
If the cheevo doesn't pop, reload the checkpoint and try again.
Took me two goes.