Pneumatic weapons are the guns that you have to pump air pressure to. Your best bet is to load up FRONTLINE and go all in on the railcar at the start of the level. Make sure that they enemies know what is going on so that they will be drawn towards you - making it easier to kill them. There should be over 30 humans to kill on this level alone.
If you don't have a pneumatic weapon you can alternatively load up the level before it called ARMORY. Here you can buy a Helsing or Tihar which both count for the achievement. Don't forget to buy ammo if you need to from the exchange kiosk.