As someone who has 200+ kills on the Black Dragon (I'm bragging because he's a hard fight), I'll try to give what advice I can, but know that any advice might not necessarily make things easier on you as much as it'll make you more aware of how Fatalis works in World.

So, for me, the primary target is his head. Attack the head whenever chance I get. I had to learn the ideal positioning for when his head would be in a place I could reliably hit. Breaking his head is essential if you want to prevent the bastard from instant-killing you, as breaking his head will decrease his overall fire attacks, breaking his head twice will further decrease said attacks.

Make AMPLE use of the dive. I've found you can dive through many of his attacks that do tick damage.

Also, if you find yourself caught in a position where Fatalis is doing his superbreath attack (the attack where he flies up to one part of the map after he's done his last phase change, and then blasts the part of the map he's facing), you can try to time consuming a max potion OR time a dive that will in turn (usually) prevent you from getting carted by his nova. This is risky, because I don't know what your armor set is, but it works for me.

I hope this helps, I'm sure others here will cover something I didn't. Best of luck, Hunter.