Solasta: Crown of the Magister

The achievement description is misleading. This is actually for getting 20 PSYCHIC damage kills. Not piercing. Once you know that it's easy to unlock.

What I did was create a custom difficulty. Lower all enemies to lowest stats possible and buff my own. Used shadow dagger with my sorcerer on the first 20 enemies of the game. Worked like a charm with all enemies at 50% health and all my rolls at +5. Works even faster with a team of multiple wizards and sorcerrors.

This difficulty trick works with all the 20 kills achievements in the game. Doesn't matter which campaign either as I got mine while doing the lost valley dlc.

This is not a solution - just caution- some of these Kill 20 enemies' achievements seem bugged. I just got this one, by doing Psychic damage.