I think the fail condition is not the real problem in this quest. If you are forced to run without a caster or if you try it solo with a melee type and hireling, off hand damage is the real issue. And I do not mean from two handed weapons either. I am talking about off hand damage from a one handed weapon. Even with targeting the orgres and making sure no spiders are between me and the target, the attacks still do damage to the spiders. I know the damage is not coming from my hireling because I very quickly learned to park the hireling far away from the fight. If there is a way of stopping the off hand damage, I would love to know. I don't mind having to be careful; I do mind quest failure for reasons that seem beyond my control.