The Lumen Flares you can pick up are a perfect way to grind out this achievement. Once thrown at enemies they will go into their vulnerable state (and stay there) allowing you multiple crackshots in a row if you keep spamming
. Once you've made your way into the open world you can find loads of Lumen Flares in the small shacks that are littered around the map.
If you find yourself in need of additional enemies to get crackshots on, utilize the side quests in the open world. The one at the top of the mountain in particular has a good amount of enemies, I believe the quest is called "The Snow Walking Dead" or something like that.