I've replayed this mission a couple of times (with my mesmer) now, because I wanted to know what was going on there.
Success. Me/Mo, my normal allround domination setup. Other team: a ritualist with henchies.
Failure. Me/Mo, my normal allround domination setup. Other team: full henchies (you know that there is no other player when the other henchies are green triangles instead of dots.) Reason for the failure: general wipeout at the second battleground.
Failure. Me/Mo, still the normal domination setup. Other team: Wa/Mo and henchies.
Failures. Me/N, having switched Echo* for Well of Power and Ether Feast for Well of Blood. Other teams: Variations of 1 A/x + henchies and 2-6 mixed players (including melee) + henchies.
Success. Me/N using Well of Power, Well of Blood, basically only Empathy and Guilt from my domination skills. Other team: full henchies. I let the henchies attack whatever they wanted and just spammed the wells and an occasional guilt and empathy. It seems that they went for the right targets. No single death.
I'm beginning to believe that it's the human melee players who cause the wipeouts. They select their targets and make the henchies attack the wrong things...
With a human other team, I've also had serious trouble with the second Am Fah ambush on the way to the meeting spot. Here, the other humans dragged my henchies away from me, so that at one time, me and Mhenlo actually died. It was an almost full human team there.
I was so glad when it was a full henchie round in the end. Now I paid attention to the story and am not that confused anymore.