Pretty much any zone you can kill things quickly that drop crystals. Set 200 mob, 500+ damage, relevant crystal(s) and go to town. Can set any additional quests that apply to whatever zone you're doing like 5 adoulin mobs or 10 insert old zone here mobs; geodes/avatarites if the zone has frequent weather. There's also monstrosity lvl capped in buburimu, though it's harder to get 500+ damage for a lot of monsters there.