Edit No. 2: The achievement will pop as soon as you equip the amulet.
Edit: A nod to Sashamorning for his comment that you should NOT punch Mr. Wiz if you want to buy the amulet from him, since you would have to start a new playthrough then to get a chance to do so.
Should be pretty obvious, but since Brick is punching nearly every NPC in this game, that behaviour could be seen as normal ;)
The amulet can be traded between players, so if a friend of you already has it, he could give it to you. Just equip it and the achievements will pop. If not, you can buy it while doing a quest. The price seems to be lower in this standalone - I only had to pay 6000 credits on level 15.
You can get the amulet from the NPC Mr. Wiz, he is a random spawn in the Lair of Infinite Agony.
If the yellow "!" for his Quest The Amulet is not on the map, just leave an re-enter the area until it is.
Zoning, fast travel and log-out and back in all will work for this.
You can do this the first time you reach the area, no need to finish the game or anything else in the area. But since he is located quite deep in the lair, you should just progress normally, doing the other sidequest on the way. You'll reach Mr. Wiz eventually after unlocking a few doors and other shortcuts, the area is quite the maze.