Here's a quick guide about how to beat Warpriest in Challenge Mode on Destiny. In order to beat this boss, you will need different aura holders with each damage phase. You can't hold the aura more than once, as doing otherwise it will prevent you from receiving the Challenge Mode drops.
You can have someone die and you don't need to beat the boss in two phases, by the way, in order to get the Challenge Mode drops. A chest spawns in order to give you 2nd loot if you killed already. You must have two different Aura holders, again; there's no exception.
As a reward you should get:
Let us know in the comment section below if you need any help on this?