Fatalis
DISCLAIMER: I am NOT responsible for any carting that may occur due to you or your friends reading this guide.
Fatalis General Tips and Guides:
Fatalis will unleash a supernova 5 times, at these health percentages 76%, 50%, 38%, 20% and 5%. The first one's safety zone will be at the far left of the arena, the second being the barricade at the back, and the other three being below him.
Have at least one person run Level 2 "Heavy Artillery" for +100% cannon / ballista damage. Especially solo their numbers are more than substantial (~22k damage in total if you do it right).
You can have exactly one player from your party eat for "Felyne Safeguard" with a voucher for an extra party-wide faint (doesn't stack).
I recommend either a combined Fire Resistance of at least 20 minimum or Level 3 Blight Immunity. The last thing you want to have to think about is the lingering fireblight effects carting you after your Moxie procs.
Breaking Fatalis' head knocks him down and weakens his fire breath in the final phase of the fight. A second break will greatly weaken it. Getting two head breaks guarantees a "Fatalis Evil Eye" quest reward even if you fail, which you will need to craft some of his equipment.
An unbroken head will give him a slight speed boost and make his flames blue in the third phase, this can lead to potential one-shots. Melee can tank some hits with a Fireproof Mantle and maxed out defense.
Watch for pods drop. Fatalis will almost always drop a pod right before he does his first and second nova where you need to hide behind the metal slag and the barrier. So it's a good tell to watch for in case you lure him too far across the map and can't make it in time to the barrier. Again, pod drops = nova very soon.
After the first nova, you can drop some smoke bombs to smoke the roaming ballista. Fatalis would land immediately not being able to see you, and the 'confused searching' animation will play. This allows for absolutely massive damage on his chest with a hit rate of nearly 100% with the ballista.
Fatalis is immune to stun effects and cannot be KO'd. He sometimes recoils exaggeratedly, but it does not result in a knockdown. However he can be paralysed and slept. Having a high resistance means you might only be able to do one status effect of each.
2 Dragon Pods will flinch him. He does NOT build up a resistance over repeated usage or time, so use this to your advantage.
Unless you play ranged or have lousy fire resistance, you can superman dive twice in a row to survive the large, continuous cone-of-fire attack (NOT the nova). If timed correctly you take only a single tick of damage (or none!) Ideally you want to be close to Fatalis' head as he does this, because it is a prime opportunity to deal free damage. Farcaster is a great option too if you can get it off in time.
Flinch-shotting an upright Fatalis will make him crawl. This won't make him hit a wall for extra damage. Similarly, flinch-shotting him mid-air will force him to land and deal minimal damage. A flinch-shot on a crawling Fatalis, however, can be directed at a wall for wall-slam damage as per usual.
Tenderising either front leg affects both front legs and the chest, so it is well worth doing. On heavy melee weapons (e.g. GS or hammer), you can make Fatalis crawl by tenderising its upper body once or twice (more in multiplayer of course). This is often easier (and safer) than trying to hit the belly ordinarily. Or just slap a Shaver jewel to your build if you're a light weapon.
Hitting the chest or front legs is easier than hitting the head. You need damage to beat the timer, so don't hesitate to go for (relatively) "safer" damage once in a while.
Whenever Fatalis gets close to a ledge, try to rack up some mounting damage. Successfully mounting him will lead to a full knockdown much like breaking one of his horns does. Glider Mantle is fantastic for this, as you can also clutch-claw onto Fatalis with it, let go, and then glide toward his head for extra aerial hits whenever you want. This isn't NECESSARY to do in a hunt, but if your group is already struggling to get damage on his head, the potential mount topple can help secure a break.
When solo or duo, give your palicos either paralysis or sleep weapons. You can trigger either status once per fight, and paralysis especially lasts for a long time. If you play ranged and have sleep ammo, give the cats paralysis and save the sleep for a Dragonator wake-up in the final phase of the fight. (Or sleep it anytime you want, up to you lol)
When solo or duo, I recommend the Shieldspire for your palico as its gadget. Fatalis going ballistic on the Shieldspire, making is a great distraction for prolonged periods of time for free damage. All other options are viable too depending on your playstyle. This is a general written guide and is meant to aid you, you do not need to follow this word for word.
PHASE ZERO: Cannons
Carry a farcaster. Equip your Ghillie Mantle before using the wingdrake. When you enter the arena, you can find two cannons on a scaffolding to your left.
Gather some slinger ammo (rocks). Load both cannons. Move the right one (facing Fatalis) one notch to the right and the left one ~70% to the right. Fire the right cannon first. Stand still so Fatalis roars and rushes at you. As he approaches, run to and fire the other cannon for ~4500 damage combined (with Heavy Artillery), knocking Fatalis down.
Jump down and capitalise on the knockdown, and be sure to clutch-claw onto Fatalis' head as he is about to recover. Wait for the animation to finish and immediately flinch-shot him into the scaffolding for another ~1200+ damage (solo). Return to camp with your farcaster and switch to your regular build.
Edit: Alternatively, skip the flinch-shot to preserve the scaffolding and the cannons. Switch your build as usual, but use a farcaster again when the game notifies you that your Ghillie Mantle has recharged. You can then repeat this process unless the fight has already advanced to phase 2 or beyond. I can't vouch for how much cannon ammo can be gathered from the piles (allegedly 4 per stack, meaning only 6 shots per cannon in total), though. Keep in mind that all players must return to camp to reset Fatalis' position in the arena.
PHASE ONE: Pre-nova
Nothing special here. Follow some of the general tips if you feel like it. You need to reach some damage threshold to trigger the first nova, which will expand the battlefield and grant access to the roaming ballista (as well as an inactive dragonator). As the nova is being charged, seek shelter behind the piece of metal indicated on the map to survive.
PHASE TWO: Post-nova
Following the nova, grab the One-Shot binder behind the metal shelter you just used to survive. Man the roaming ballista immediately and move it all the way to the left, but do not fire it yet. Unman the roaming ballista. Tenderise Fatalis' chest if possible and wall-slam it if it is not yet enraged.
Wait until Fatalis is busy trying to cart something a good distance away from the roaming ballista, man the regular ballista next to the roaming one you previously moved and wait for Fatalis to turn and (roughly) face its direction. As soon as he does, fire the binder, rush to and man the roaming ballista, then empty its whole magazine into his chest. This deals ~90 damage per shot (~9000 damage in total) with two ranks in Heavy Artillery. If you have trouble using the roaming ballista safely, bring and use your Temporal Mantle as you man it.
Make sure to fully deplete the roaming ballista's ammo. The game will notify you when the roaming ballista is about to restock itself. Beyond this, proceed normally until you've dealt enough damage to commence the final phase of the encounter. Should the roaming ballista recharge before you reach that phase, leave it be for now. With enough damage dealt, Fatalis will attempt another nova. Make a mad dash to the platform behind the Dragonator and pull the lever in time to survive.
PHASE THREE: "Supercharged" Fatalis
Fatalis now glows red and his fire attacks will likely one-shot you until you break at least one of his horns. He also gains access to a 360-degree fire breath which is best avoided by hugging his chest / belly or clutch-clawing onto him (hitbox RNG is still a thing, however).
Fatalis gains a new pin attack, in which he will try to press and melt you to his body. This has a big tell and can be easily avoided as long as you don't overcommit. If a teammate gets stuck, pod him twice to flinch him out and save your buddy.
Collect the One-shot binder near the regular ballista on the opposite side of the roaming one. As soon as the roaming ballista is restocked, proceed as you did in Phase 2 to get more free damage on Fatalis.
When the Dragonator becomes available, stand in front of it, not on top, or else Fatalis will mostly shoot fireballs at you. He usually will do 4 attacks before coming to you. As soon as he crawls toward you, run up to the lever and time it such that both spikes connect with Fatalis. Solo, each Dragonator spike deals 3300 damage for up to 6600 damage in total. A Dragonator hit always causes a knockdown for free follow-up damage, so be prepared to jump down and capitalise on it. If you have access to sleep ammo, try sleeping Fatalis in front of the Dragonator for a massive Dragonator wake-up instead.
Any novas in this phase will have to be avoided by running "behind" Fatalis as he charges up the attack. As the nova's flames cover roughly the shape of a triangle with its peak at Fatalis' position, it's usually shortest (and thus safest) to run somewhat diagonally to his left or right side instead of directly toward him. Fatalis will attempt 3 such novas (again tied to damage thresholds) in this phase before he finally dies.