[Balanor]

Damage Resistance will negate subdual and physical damage and Damage Reduction may cause certain level weapons not to work. It's likely this individual may have had an item or ability that provided one of those. But as Hellstrom mentioned and I'll just point out - who says these NPCs are civilians? Mercenaries and adventurers are quite common in Thay. Maybe there is a very good reason the person you took the quest from couldn't retrieve the item on their own. :)

However, to answer your question - no, bribing the NPC is not the only way to get the item. There are multiple methods by which you can obtain the item for free. PCs with certain skills and abilities will have an easier time of this than others. While the target individuals are not indestructible, the method you selected in this case is meant to be the most difficult way to get your object item. Other than the IC reason for the target NPC's power, the OOC reason they are one helluva fight is because this object retrieval quest type is not meant to be a glorified assassination mission. The difficulty in fighting them is meant to reinforce that you should really exhaust any and all other opportunities (use certain skill-enhancing spells and items prior to talking to the target, engage your rogue allies if you still need assistance, among other options too) before taking the path of least resistance and attacking them.