Firstly, as an assassin, I wouldn't really recommend that you hench the mission as you will be required to carry the urn.
Secondly, I recommend you take the middle path, much quicker (MHO).
Thirdly I'd say that a degen or interrupt build would probably be good - though I don't know much about specific assassin builds. Make sure you have a source of healing too, as monks find this mission particularly hard being interrupted all the time.
Any strategies that can help?
Lady Lozza
AW Lore
i henchied the mission with my assasin 
, it went a bit harder than expected, i went slowly and carefully, once you get to the urn and the falling debris, i would go afk while in a safe zone to get the urn to lvl 5, then move to a where the spawn of onis are, drop it before they appear, then wish i could kill them faster than they could kill my henchies.
if you are a a/mo, bring a monk healing spell to help your henchies and Danika, i went with jamei gaze.
there are 4 oni spawns if you go the left way.
one is near the first set of stairs once you go the left way, before reaching the grass like monsters.
after walking two areas of "green grass floor", you will come across very big stiars, there are two oni spawns there, one at the middle of the stairs, and another at the end, near the corridor you have to take to the right.
there is anoter oni group further ahead, , between two zones of "falling debris" enviromental effect.
the last oni group will be at the end of this corridor.
through this path, the only real threats are the onis... and the large group of monsters at the end.
when reaching the end, leave the judge for last, its a warrior boss, once you kill him more monsters will spawn.
try to kill the stonesoul first, as he is a monk and can heal through his divine favor.
good luck
, it went a bit harder than expected, i went slowly and carefully, once you get to the urn and the falling debris, i would go afk while in a safe zone to get the urn to lvl 5, then move to a where the spawn of onis are, drop it before they appear, then wish i could kill them faster than they could kill my henchies.if you are a a/mo, bring a monk healing spell to help your henchies and Danika, i went with jamei gaze.
there are 4 oni spawns if you go the left way.
one is near the first set of stairs once you go the left way, before reaching the grass like monsters.
after walking two areas of "green grass floor", you will come across very big stiars, there are two oni spawns there, one at the middle of the stairs, and another at the end, near the corridor you have to take to the right.
there is anoter oni group further ahead, , between two zones of "falling debris" enviromental effect.
the last oni group will be at the end of this corridor.
through this path, the only real threats are the onis... and the large group of monsters at the end.
when reaching the end, leave the judge for last, its a warrior boss, once you kill him more monsters will spawn.
try to kill the stonesoul first, as he is a monk and can heal through his divine favor.
good luck