Path of Exile

While there is no sure fire way to make sure that an enemy drops a unique item, gold amulets and gold rings will increase the rarity of items that drop. For a time scale reference, my first unique dropped when I was level 24, and my second one dropped when I defeated the final boss of Act 2.

Other ways of obtaining unique items are by buying one from the trade market, trading with another player/friend directly, or using an Orb of Chance on a normal quality item (this is very unlikely though and is actually a separate achievement).

Once you have a unique item and equip it, the achievement will pop. As a side note, unique jewels (the ones that go in the jewel slots in the passive skill tree) do not trigger this achievement.