One of the most annoying things is creeping up on a Imperial camp, ready to massacre all of the residence, all except for the unkillable captain dude. Is there any way to make him killable?

Not in vanila version.

There are PC mods that will allow you to kill them, but when you are sent there to kill them as part of a quest, and can't get the quest to progress... no complaining about breaking it... o k?