Head to the Lair of Infinite Agony and you can find a mission called 'The Amulet'. This is given to you by an NPC known as Mr. Miz. There are reports of Mr. Miz not being in the Lair* (though for me he appeared the first time entering just after beating this DLC story and before the Raid Boss). Just exit and re-enter the area and he may spawn. If you see a yellow exclamation point, check it out.
Players have reported seeing him before finishing the story, so once you come across this area it can be done at any time.
Accept the mission and you'll have the option to buy the amulet or punch him. You must have at bare minimum 125k cash to buy it in Normal mode. TVHM the cost is around 850k. If you're on UVHM it will cost you anywhere from 3.3 million to 13 million (at level 72). Overall if you are wealthy you will be fine. After you buy the amulet the achievement will unlock once you equip it.
If you punch him,
Alternatively, if you accidently punch him or are short on cash, you can get the amulet from a friend by trade/drop. Just make sure you're the required level to wield it.
*According to the Borderlands Wiki, Mr. Miz is confirmed to be a random spawning NPC.
-Credit to Helmaroc for the cost of the amulet in True Vault Hunter Mode and the dashboarding technique.
-Credit to spacedvest2 for the trade method.