Well I thought I'd shed some light on this guy for people who plan on killing him in the near future. First of I've lost about 20k exp fighting this guy, some people have had it waaaaaaaaaaaaaaaaay easier then I have, so my hats of to them(Wiped 2 times at 1%). But for those like me, well I'll tell you what I know and perhaps it will get you the win.
First off the set up:
Stunners are a must, their is no way around it. Tanks, well it's really up to you and what you can get, Plds work well, Nins to well also, hell even Pld/nin works. Or even having a Pld and a Nin. From what I've heard the Pld Whm combo works great. Now feel in the DD spots of course most of you will have smns along Blms are helpful as well. I would assume, though I haven't tried it, that bringing other DDs like mnk sam War etc etc that the only stun capibilty is a WS could work, but make sure you have enough stunners. I normally go as Rdm/Drk. Your main focus is to just stop Nightmare, nothing else.
Now once you have damaged him to 50% He will SPAM, let me repeat that SPAM Nightmare. At 40% He will use his 2hr. Now here is the kicker. At 50% or less I would CS stun, I stunned countless Nightmares and his 2hr, BUT he resisted one of my stuns and got a nightmare off. But thats not the kicker, what he did after was his 2hr AGIAN, even though I had previously stunned it before, now for me I hope thats a glitch, and as soon as I learn how to post Screenshots, I can show you. So what I'm saying is, pretty much at 50% you have to kill him FAST, basically go all out. Also you have to make sure his Magic Def Bonus is down, or he resist a Rdms stun... wait no, he just laughs at you.
Anyway, this was sort've a rant/guide, feel free to add info if you wish and I'll get on that Screen shot posting.