Not too difficult a quest. Set up spirits at the North end and then flag heroes to the South end. You should have plenty of time to get there, after which you should be able to kill all the spawns quickly enough that the spirits live. Cast as many Ebon Vanguard Assassin Support as you can, together with "You Move Like A Dwarf!". Focus targets down quickly with the massed ranged damage you have (the monsters have no hex removal or interrupts, so you can freely cast Assassin's Promise), and pay attention to the spawns on both sides because the north spawn will not be delayed indefinitely. Be ready to micro heals onto the spirits if you have to. Good luck!