Jack is planning on building his own arena after destroying Moxxi’s, but she has a better idea. Head out to Opportunity and go to the marker to find Foreman Jasper, who is pretty much nothing but a stronger Engineer. Kill him and he’ll drop a key, so pick it up and head towards the next waypoint.

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Foreman Jasper is basically a Badass Engineer (left). The container you need (right) is pretty easy to find.

The container for the key you picked up is located on the upper level and once you open it, you will find some explosives. Pick up one of them (you don’t need both) and exit the container, only to be greeted by a Constructor and some Loaders. After dealing with them, go to the next waypoint and you’ll find the dam where you can place the explosive. Do that and step away, waiting for the explosion. Return to Moxxi to finish the mission.