I struggled a lot with this fight and had to look up some strategies and those are the things which worked for me:
1- keeping the archer alive
2- Taking out the ads
3- When he finally transforms I just kited him. I let npc kill him lol. I just ran around, threw in some dps, healed npc. They finished him really fast.
4- At this point only ads were left and the only npc I lost was the healer, burned them down and won
I'm a whm btw.