How to beat him for challenge mode?
It looks like the only difference is that you need a different aura holder ever time. Deaths and wipes appear to be fine. You don't have to beat him in 2 auras as people have believed, just have different brand holders for each damage phase.
Rewards
Normal mode challenge gives 310 stuff including artifacts and the fragment (no emblem)
Hard mode challenge gives 320 stuff including artifact and emblem and Calcified Fragment
Can I do it on multiple characters?
Yes.
If I fail the challenge can I go back, kill him properly, and get the challenge rewards?
Yes. They will spawn in a separate chest.
If anyone has additional information let me know and I'll add it to the OP.
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Bumping this because it's relevant again