This can be unlocked in the "Well of Shadows" dungeon in the new Starving Sea area of the map. After unlocking the Lobb Shot and then progressing to the end of the dungeon, you will come to a large open area where you will need to use the Lobb Shot to kill enemies behind a shield. This is a good area to unlock this achievement because the enemies are trapped where they are, and you can back up to the other side of the room, get the aim right, and shoot at them as many times as you need to.
To switch to the Lobb Shot, press a direction on the D-pad until the correct icon comes up (colour match to the enemy), then hold
to charge up a shot. Aim high up above an enemy from the other side of the room and shoot until it dies, just make sure the shot spends at least 3 seconds in the air.