I’m surprised to read this. I played on expert and didn’t recall this fight, only the (mediocre) cinematic.
So I looked online, and apparently he can use all sorts of cool abilities, like spawning clones and stuff. I might’ve killed him too fast for him to do any of that 
The only boss that was overtuned for me was The Beast from the SB class quest.
I wouldn’t necessarily agree about the spiritborn. I’m not following any build, I tend not to, and I’m not having a problem mowing down mobs on Penitent, and I’ve been on that difficulty since very early in the game. Granted Ah Bulan did spike the difficulty, but even since then I’ve gotten so much gear that have boosted the direction I want go to.
I’d imagine its likely down to a build not being too good, or you just don’t have the best/right specs/stats/affixs for the build itself.
If there’s going to be any changes to the spiritborn in the coming seasons, it’ll likely be a nerf instead as I see it as being pretty strong.
As for the Ah Bulan fight itself, there was one particular move that would one shot me, it was a long sweeping swipe, just gotta watch the visual cues and move in and out when attacking.
Issue isn’t with the build, but the difficulty spike. I have most of necessary legendary effects for my build, fully geared up for my level. Tried Ah Bulan even on NORMAL (supposed to be an easy mode, cause I got really frustrated with repeated attempts at 2nd and 3rd difficulty) - no dice.
One shot move literally has no indicating effects or signals - it just happens. Moment ago you were at full health and then… BAM! Instakill. Brilliant.
NEVER IN MY LIFE had I issues with Diablo games prior. Blizz messed up and this needs to be patched ASAP.
The class doesn’t need any nerf as it’s as weak as a wet paper. It’s the all classes across the board who need buffs after “stat squish” Blizz commited.