Rather than view the Lucian encounter as the boss fight, view it as an interactive cut scene. Reaver's Rear Passage was your hordes of faceless minions you needed to cut your way through, and the giant shard was your 'final' boss fight.
Put it this way, from what I saw the only 'power' Lucian had was to make you sleep, obviously he can be killed by one bullet cause that's what you do. It wouldn't have made sense to have this flimsy character as your last boss, he was just a dude who pissed you off and had to die.
I loved the ending. Was full of story and very dramatic, different in a good way. Plus it's really quite satisfying pulling the trigger before he gets a word out. Having to listen to antagonists blurt out all their plans and personal emo life stories is a mood dampener. Just boom and down he goes.