If like many players you didn't get the chance to shoot enough vortex horrors in the Anomalous Cave quest, and after completing the DLC story you found their spawn rate is extremely low, you still have a chance to get this achievement easily.
Check if you have the ingredients to craft one of the 5 new grenades introduced in the DLC, called Vortex Grenade - Lure. This grenade spawns a vortex horror.
If you can, craft one and equip it on your companion and go fighting. Your companion will use the grenade and you can shoot the vortex horrors spawned by it, and they count toward the achievement.
I was able to test and confirm this today to get this achievement.