Before continuing, this achievement seems to be bugged. Just completed the fight but it didn't unlock. Anyways, here's my Strat.
This is all about timing and dodging.
The ONLY time to attack this guy is when he flashed green. Otherwise, hitting him will drain your health and heal him. I know right, seems rude. Also, if you get hit, it'll leech your health again. So basically, only hit him when he flashes green, and dodge his shield rush and sword attacks. Essentially treat him like an even worse marauder.
When the wolves come, kill them. 2 of them give you a hammer strike which elongates the stun time. You'll need this to get through it, or by the time you drain any health he will heal again. It's pretty much the same each phase of the battle, except he adds a move each time. Phase one is basic, phase 2 adds a bomb/grenade, and phase 3 sucks as he can spawn in ghosts of all the worst demons to fight so you're essentially fighting him.and 4 heavies. Ignore the heavies of you can, cush him so his sword comes out and when he flashes green, shoot to stun, hammer, which should kill the heavies, then nail him.
So here's the simplified version: Equip super shotgun, sut back and wait for wolves, take out the 2 wolves, run towards him but be careful and be ready to dodge, when he flashes green, fire your super shotgun at him then immediately hammer strike him, hut him with 4 blasts of the shotgun and back off, then repeat. Ignore the bombs and heavy enemies in phase 3 where possible.
Any questions or anything I've missed, hit up the comments below