MISSABLE: Don't punch Mr. Miz!
If you already punched Mr. Miz, check out the Achievement Trading thread or find a friend that DIDN'T punch him who will drop it so you can pick it up. Achievement unlocks when you equip the amulet.
Mr. Miz may or may not show up during your initial run through. Many have reported not being able to find the guy. He appears to be a random spawn like the Goliath that drops an echo recorder in The Fridge (side quest from the main game). After the campaign is finished there will be an "Undiscovered" quest in the Lair of Infinite Agony to point you toward him. If he's there, you should see his exclamation point on the map. If you don't see his exclamation point, just fast travel away and back until he pops up. You must purchase the amulet. It is expensive but the price appears to vary; it's unclear exactly what all the price is dependent on. It cost me 750,000 on tvhm. It can be duplicated.