There are various ways to perform environmental kills:
- Ignite swamp gas with Igni.
- Shoot explosive barrels nearby wounded opponents.
- Cast Aard on an enemy and move in for an insta-kill.
- Shoot harpies with the crossbow and finish them when they are on the ground.
I unlocked this achievement by playing normally by the time I arrived in Skellige, so there might not be a need to grind this if you are not in a hurry. Also, it's been said that the Battle of Kaer Morhen later in the game grants a number of environmental kills via meteorites and explosions.