Just did the Exo Challenge solo, when I got to the "Boss Phase" (destroying the 4 boxes to access the Vex Hydra) the boss and adds spawned in and instantly died, instantly finishing the Exo Challenge.
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[quote]Just did the Exo Challenge solo, when I got to the "Boss Phase" (destroying the 4 boxes to access the Vex Hydra) the boss and adds spawned in and instantly died, instantly finishing the Exo Challenge.[/quote] Because you destroyed the large cube in the middle. That ends the encounter.