Cash Out

Cash Out is a challenge and achievement/trophy in the Fallout: New Vegas add-on Dead Money.

Walkthrough
Once you're inside the Sierra Madre Vault during the quest Heist of the Centuries and you have made Elijah come down to the vault, wait inside for him to stand in front of the door or when he comes from behind the shield to kill you, then is your chance to kill him. The achievement will appear after he is dead.