Once you beat the game there is a new Sidequerst on the Concordia bounty board called The bestest story ever told.
You need to kill that boss, that you just kill again, only slightly harder. 20 Moonstones is required to fight this raid boss.
It is not that hard, there are always spawn that you can kill to get a second wind! . I beat the him right with my level 29 Fragtrap.
– After completion of the main story, the Sentinel can be fought repeatedly in its original form, by returning to the Vault in Eleseer.
– The Sentinel can drop the The ZX-1 (legendary laser)