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. When everyone is dealt with you can reload checkpoint as this doesn't reset your progress.
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 which counts for the achievement. Don't forget to buy ammo if you need to from the exchange kiosk.
The mentioned spot is great for most other cumulative achievements: