Solasta: Crown of the Magister

The easiest way I found to do this was to take a character that can cast fly to the Library in the Manacalon Ruins. Fly a character to any of the middle platforms, but when they are mid flight over the abyss, pause the game and click the "drop concentration" (blue circle icon on the casters portrait). The spell ends and they will fall to their death which unlocks the achievement.

note: A wizard can learn the 3rd level spell Fly at 5th level. You may have access through random loot to a fly scroll or potion, or a magic item that grants flying but I'm unsure if these allow you to drop the concentration on demand.

As this can be a bit of a pain to get with the AI enemies I found another way.

Cast fly on a low health character.

Fly them as high as they can go

Save (in case of low damage roll you can reload)

Cancel concentration on fly and watch them fall.

as long as the fall damage takes them to 0 hp this will unlock.