Just outside the USS Constitution there's a brotherhood lance initiate that vere rarely moves, doesn't attack, and cannot be killed. It's preventing me from finishing the last voyage quest because I need to speak with the navigator and I can't while he's in combat (which is forever, due to the enemy's invulnerability). I've searched around the Internet for a solution and the only one I can find is to use commands, which I can't because I'm on Xbox. Fix this with haste please.

Just a thought, but have you tried going somewhere else, waiting a few days in-game (maybe 3 or 4) and seeing if he disappears or respawns (which might "reset" him and fix the invincibility)?

If not, I'd try that and see if that happens to work. It might not, but its worth a shot.