For me this achievement unlocked as I killed a normal enemy and it dropped a green looking item, upon picking it up the achievement unlocked a second or two later.
I know a few people who where killing the unique enemies had one drop from them.
I asked for some further details and it seems all of are luck status where above 10 at the time, since luck affects your drop rate, I'd recommend getting it to 10 and then continuing with the story or farm a bunch of enemies. One of my friends did say that they had a accessory equipped that increased the drop rate for rare items. (no clue where he picked it up from tho) Also note if it drops over a cliff or into a gap where you'd die if you where to jump down to it, once it hits the floor it'll be automatically added to your inventory and will still unlock the achievement.