Solasta: Crown of the Magister

This achievement is for dealing damage to a fellow party member with a spell. So during your playthrough if you have any character with a damaging spell: make a save then just cast it in the direction of an ally. AOE spells that wizards have make this simple. Once an ally takes damage the achievement will pop. Afterwards just reload your save so you don't stay damaged.
I did this with burning hands and it took me like 5 seconds.

If there is anything I can add to help or perhaps any way I can make this guide easier just leave a comment.