You'll run into Nosalis monsters in the 3rd act of the story, after you've
. You'll get to a door that needs power to be cranked up, but in typical Metro fashion, the system will need to reboot and you'll have to defend yourself against these beasts until the door is fully open. Killing one with the butt of the flamethrower is pretty easy on Normal. A few hits and they'll go down. You might die in the process of getting the 3 kills, but that's ok. If you get the 3rd kill and you've taken some serious damage, just reload your last save so you can start the fight over with full strength. Then toast those fuckers with the Grillmaster 9000 and move on.