For the fatalis, I did it with my friends. The only strategy I can suggest is to go in with the best dragon weapon you possibly can, and bring a fully upgraded armor that compliments your weapon. For example, I used silver rathalos armor with element attack up, pierce shot up, and attack boost M with the akantor bow, and my friend used full death stench for the ESP with the gun chariot gunlance. Stay far away from him when he flies, as his tail can whip you into range of his fireballs. Guide him under the castle gate to make it fall on him for massive damage, and try to keep behind him, hacking away at his legs. After 3-4 quests you should kill, but I don't suggest making his armor, it sucks out loud ;)