Context: You want to hunt Fatalis but you don't want to bother people asking for help and you only know how to play 2 weapons in the game which are great swords and hunting horns...

This is a vanilla game and no mods whatsoever

After 20+ attempts, I'm quite confident to say that the crucial factors for successful runs are as follows:

1. FPS - I set my graphics as follows to achieve as stable and as close to 144 FPS as I can. Here's how I changed my graphics settings:
- Set FPS to 240
- Turn off V-Sync
- In advanced settings, turn everything down to low and turn everything off except z-pass (I get consistent results when leaving this last one on)

2. Meowcano glitch using a snowman + 2 mega barrel bombs placed in front of Fatalis (preferably right in front of those hanging hands where they're not colliding with your hitbox)
Then try to position the Palico inside of them so that the meowcano can glitch through all the objects and whistle away the madness upon Fatalis

3. Stay undetected throughout the meowcano duration (the damage glitch actually comes from the final explosion) In the runs, I use flash pods to prevent Fatalis from doing random attacks (I'm still not sure what and how exactly to stun lock him because even with flash pods he sometimes still spits out random fire breath) Keep moving to easily dodge the attacks when they come. As long as you don't get hit by his attacks, you'll still be undetected even if he's still shooting.

And if for some reason the barrel bombs and/or the snowman got destroyed before the final explosion, you can consider that a failed 1st meowcano which I would recommend restarting the mission because the hassle of trying to redeem it won't yield any benefit from that point.
If you can place them perfectly and not have any frame drops when the last explosion happens, you can actually skip into the melting barricade animation and then finish him off with the roaming ballista or dragonator
Here's the sample of skipping 2 phases    • Fatalis Meowcano Max FPS Cheese Done ...  

Closing thoughts:
I would say this is a fun technique to try if you already been beaten so many times by Fatalis and still can't learn his moves because the attacks are so punishing. At the end of these runs, I find myself able to handle his attacks more properly because when I run these meowcanoes, I don't have to do much of the attacks and get to observe more of his moves.
And finally, a normal run would be a lot easier and less stressful in my opinion so, if you have friends to play with, just run with them :D