Monster Hunter World: Iceborne - MR6✪: The Black Dragon, Intro Cutscenes & Solo section
Fatalis first run. Bring a farcaster - they work in this quest, unlike in Alan. After successfully reaching the initial repel, the instance becomes active for SoS, (and for every attempt following. You'll then start in the camp on future attempts.)
For the second phase, Fatty has a couple of AoE hurtbox events. Hide behind the blocks dotted around the arena for the first. There is a barricade at the back for the second - be sure to close the door once you're all in there. Just run for it, btw, despite the fire - you'll make it as long as you weren't fighting him at the far end. For the following events, gather under and slightly behind Fatty - his attacks in the third and following events only hit the ground in front of him.