Wield The Mysterious Amulet

This achievement can be done after completing the story of the DLC and before the raid boss, as I got it before killing the raid boss.

After you complete the story of the DLC, 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 after beating the story). Just exit and re-enter the area and he may spawn. If you see a yellow exclamation point, check it out.

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. If you're on UVHM it will cost you 3.3 million, so if are wealthy you will be fine. After you buy the amulet the achievement will unlock once you equip it.

If you punch him,
he will call you a d*ck and the mission ends so don't do that if you want the achievement.