How do I get the "Ethically Questionable" achievement in Black Mesa?

The Ethically Questionable achievement comprises of the following actions and occur during the 'Questionably Ethical' chapter;

• Killing an alien grunt with poisonous gas
• Using the tesla discharge on the bullsquid
• Using poisonous gas on the headcrabs in their habitat
• Shooting the headcrabs with a laser
• Firing the snark cannon until it breaks

As mentioned by PWhite these are all segmented off in their own separate rooms where you're generally presented with a button to press to activate lasers.