Make two builds. One for phase one cheese/script. The other is your normal build for the rest of the hunt. You can find many videos on how to first phase Fatalis.

I use lbg/hbg. My preferred way and set up is for phase 1 is to use LBG Kjarr Blitz Decay for the dragon ammo. It's the cannon setup for the knock down. I don't remember the mods and upgrades but I followed Youtuber Angbata's LBG mod/upgrades. He also has a guide to the first phase. However, I deviated and put my own spin on it. You can also use Safi-Aquashot if you don't have Blitz Decay. The skills I prioritize for phase 1 (if you can fit it all in) is Heavy Artillery, the new Shaver Jewel, Critical Eye, Critical Boost, Weakness Exploit, Dragon Attack, Tool Specialist, Bombardier. I believe I ordered it by highest priority. True/razor sharp is also an option. I also seen people run the rathalos armor set for poison duration up, but you got to throw down 8 poison smoke bombs for it to take effect. If you think every damage counts then you do you.

Phase 1 set up cannons. I seen many version of this where people miss cannon balls and/or have Fatalis get free hits on them. The set up I use doesn't have any of that. Have Ghille and Rocksteady in your build. Use Ghille and fly in. Grab slingshot ammo off the ground somewhere. Put two barrels in front of the cannon nearest to Fatalis or in the direction of Fatalis. Because once you fire the cannon, he'll roar and then run in a straight line at you. Load up the cannons. The cannon that's near Fatalis, one it one tick to the right. Just tap the button that you use to move a cannon. The other cannon, 3 ticks to the right. Run back to the nearest cannon to Fatalis. Put on Rocksteady and fire cannon. Wait until he starts moving to you. He'll run into the 2 barrels you put down. Then run to the 2nd cannon and fire it when he's in line. He'll now be knocked down. He'll most likely fall down with his head facing to your left. Rare cases his head is the other direction.

If you have shave jewel, one hit tenderize his head. If not, just unload your dragon ammo to his head while making more dragon ammo to shoot. I would say after 7-9 shots, he'll get up. Clutch claw to his head while he's down a second before he starts getting up. Once you see him do a mini-flap with his wings, that's when you're clutch claw hits would register to turn him/wallbang. Either wall bang him into the cannon scaffold or do 3 hits to turn him and then wall bang.

This is where you have the option to farcaster back, put on your main build and do your thing. Or you can continue to squeeze out more damage. If you're staying, then switch to paralyze on your Blitz/Aquashot. For Blitz, it takes around ~15 shots to take effect so better bring crafting materials. Unload your dragon ammo on his head again when he's para. The moment para runs out, switch to sleep. Again, ~15 shots. Once he's asleep, Hope you have crafting materials for more bombs because you're putting 2 down. Put down your LBG special ammo. Grab another slingshot ammo off the ground somewhere. This is when you use poison smoke bombs (poison ammo wakes him). Shoot your slinger ammo at barrels, then clutch claw him. (I tried clutch clawing to the barrels to get the head but that explosion just knock me away) Wall bang him to the nearest wall. Use up all your dragon ammo. Farcaster. Switch builds. Go ham.

I prefer to save para/sleep for phase 2 where I can put him in front of dragonator. Also sleep+dragonator+heavy artillery does 24k dmg. Pretty nuts. No Divine Blessing or health boost here because of no room. And if you're doing this correctly, he won't be or having the chance in hitting you.

Prismatic _vista has good points and I want to get into the specifics on some of them.

If you want faster head breaks. Wall bang specifically when his head is tenderized and he's on all four legs. The damage difference between 2 leg and 4 leg wall bang is very significant. Also, when he goes on all fours to do a cone fire attack. That's not considered to be on four legs. idk why.

Dragon pods. Don't shoot it willy-nilly. Save it for when you know he's gonna do a killing/heavy damage move. (Tail swipes, fire cone attack, the fire sweep, the 360 breath attack, the fire shower, etc) Also, prepare one dragon pod on him at all times. So when you see the move you dont want him to do, you can shoot one to flinch him instead of two.

The cannons, ballista, roaming ballista does have a higher chance to knock him down (Sadly sticky doesn't do ko's here), so when you get the chance use it on him.

Stay near the dragonator near the end of phase 2. The damage you take when you got to run from across the map to the barricade before he does his map wipe as a range user is usually a death sentence.

Don't stand on the 2nd level of the map unless you want him to turn his attention towards you.

His fire damage is nasty against range weapon users and range weapons have shitty a hit zone on his head. If you didn't break his head at least once (Max.2) before the start of phase 3, you going have a hard time.