Alatreon is a fun hunt, or so i've heard. You fight him in Ingle Isle, on a plateau overseeing the ocean. It's hot so bring some cold drinks and prepare yourself. I've never fought him during my time in 3U and Generations, 3U had a bit of a climb to get to him, and i've never had the time to climb ranks to fight him in Generations. With that logic, I HAVE NO ADVICE ON HOW TO FIGHT HIM.