Score top tier loot in Proving Grounds and the return of Circle of Slaughter arenas.
Quick info on that Proving Grounds:
> Proving Grounds: themed gauntlets guarded by an Eridian known as the Overseer who wishes to test your mettle on behalf of the mysterious “Masters.” Proving Grounds are races against the clock, tasking you with blitzing your way through three areas filled with baddies and beating a boss in 30 minutes or less. If you complete your chosen Trial quickly and efficiently you’ll earn better bounties, and for even greater rewards you can complete bonus objectives like beating the Proving Grounds without dying, completing it with a certain amount of time remaining, or managing to find and eliminate an elusive Fallen Guardian who roams the area. The chest that awaits at the end of the Proving Grounds always contains valuable loot, but you’ll need to display true mastery if you want the best rewards.