Re: the spoiler, here’s how it works, along with the tactics I used to beat him.
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The music box will stun him 3 times. It’s up to you when to use it. I recommend saving it until at least after he switches into two-handed mode with his axe.
It WILL work on him after he transforms, but only until he howls. Ideally, you want to use two shots on him in 2-handed mode, and one immediately after he transforms.
What I would recommend is this… Use the hunter’s axe, in single handed mode. Try to riposte him with your gun combo, as this will do massive damage to him. Then, once he switches to two handed mode, use the music box (make sure you have room, as it will NOT interrupt his attack. Put some obstacle between you and him when you use it, but also be close enough to move in on him fast). Then, when he’s stunned, go up to him and hit him with the heavy attack, which will come downward and knock him down. Then immediately charge up the heavy attack, and he’ll get up and immediately get slammed down hard. Then back off and repeat. This will usually end up doing enough damage to cause him to transform.
Once he transforms, you get get the music box in one more time, and do the same move.
Then, it becomes a race to beat him down, but at this point he’s super fast and does HUGE damage. You cannot effectively run away from him, and at range he’ll wreck you with sweeping attacks you can’t sidestep away from. So, what you want to do is constantly dodge TOWARDS him, which will end up putting you behind him. Dive at him, hack at his back, then dive towards him again, hack at his back, repeat until he is dead.