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Achieve a score of 70,000 or higher in the arena.

For this achievement, you must play Endless Mode in the Arena, which is unlocked after beating the game once and completing Challenges XVI through XX. Once you have access to Endless mode, you'll need about 75 waves worth of enemies to get the required score. I would suggest doing this after your Apocalyptic playthrough, that way you have a very high gear score (1300+) and every upgrade/skill available. See "The Promised End" for info on how to build your character.

Every five waves you'll get a boss, and in the higher waves there will be multiple bosses at the same time. Be sure to save your ultimate attack and a few bars of Wrath to use shadow clones to help with these fights. However, strafing around the edge of the circle with Strife for over an hour is pretty much guaranteed success for this achievement. A higher gear score is nice, but not entirely required.