This is more a bug then an exploit, and it requires a rogue in the group
and i dunno if it has been posted before, didnt find anything when i searched.
At the last boss, there is an NPC in a cage, when you "Free" him, 3 ads will come.
They might seem as they are in combat, since they are moving so fast, but they are not.
Ask all party members but the rogue to stand at the cage, and open it, when the mobs come, distract them, and let them spot you (Remove you from stealth)
after that vanish.
This will result in that the mobs wont attack you if you attack them, they will just wander around untill they get killed
Not the most usefull bug, but its quite fun
Peace