Solasta: Crown of the Magister

This can be easily done by having your team ready ranged attacks. When an enemy moves into line of sight, your characters will fire at it, dealing damage if they hit. You do not need to kill the enemy in one hit, nor do you need to kill it in one turn. You can stretch the damage over the whole battle, but as long as you kill it with a readied attack the achievement should pop.

Note: You can ready melee or cantrip attacks, but melee requires the enemy to be adjacent to you, which means that if your team is split up, you won't deal as much damage. Cantrip attacks are great, and you should use them if they deal more damage than your ranged attack, as you would not need to expend any ammo.