So it's FATALIS time

So I've fought FATALIS a couple of times and I can't help but notice a very large detail he has where one attack nearly kills you, I don't exactly have his whole moveset down but I'm trying my best to learn, my defense is 954, fire RES is 22 and a abundance of skills at my disposal so I would assume I'd be able to shake a hit off here and there but nearly every time I get hit I have to back off and heal but by the time I'm done healing he's already looking at me charging up a fire ball that insta carts me, so far every mission I've done ended with me dying 7 TIMES, that's including vigorwasp revival and felyine safeguard, it's ALATREON all over again, do I just need a more optimised build? Or his the whole goal just to not get hit