This challenge is available at the start of the Eleseer map, and can only be completed during the main story playthrough during the mission The Beginning of the End with the objective "Find the Vault Entrance". At the first section of the map, an Opha must be killed before a Putti activates the three Onager turrets above the door leading towards the central area.
A possible strategy is to lure the Opha away from the door and attack it from a position closer to the Fast Travel Station. However, a better way to attempt this challenge is by sniping the Opha from the eastern ledge where the single Cult of the Vault symbol is located as the Putti fly towards Vault Hunters in a direction away from the door.
- This challenge will not trigger if the mission Z8N-TP has been accepted but not finished. Vault Hunters can enter Eleseer, take what's needed for Z8N-TP, turn in that mission, and then come back to Eleseer to get the challenge so long as they haven't progressed further in The Beginning of the End, though. A quit/reload will probably be required so that the necessary Opha respawns.