Zaishen Challenge is trivial to do alone. I used an EA fire ele with the W, Mo, E henchmen against the IW mesmers. People seeking to farm these missions efficiently used trap rangers, though it hardly matters if you're just trying to unlock the Elite outpost.
Per the wiki, unlocking Zaishen Elite requires completing the The Path of the Zaishen Elite quest and maxing out the daily Balthazar faction limit (2000) for the Challenge on one occasion.
