Actually, it's more like this Elemmire
wwDww
MXOOX
XXXXX
XOXOO
XXOTX
X= not safe tiles and
O= safe tiles
D= Diabolos start
M= Mages stand
T= Tanks (or other melee)
w=wall (basically where he starts... the portal thingy)
Standing like that, mages won't get hit by nightmare and can wake the tanks and melees up. And it wasn't my idea

It was everyone in the group's idea as we talked about the strategy several times and where to stand. Like I've always said:
When trying something new, never expect to win the first two times. Se them purely as training to make yourself familiar with the battleground and enemy moves, strenghts and weaknesses
Basically, it's all based on "Third time's the charm" XD. Either way, our set up was BRD RDM WHM BLM RNG NIN, where the most important jobs are NIN, WHM and BLM, then either RDM or BRD as a refresher, The last two jobs can be anything. Reason we had a BRD and RDM was for increased dark resistance (which became useless as mages could easily avoid Nightmare), and RDM for stun (which was also useless, as Nightmare could, once again, be avoided).
What you do basically, is have mages hug eachother in the corner so they're out of range, while ninja bink-tank Diabolos. Black mage nuke him with Thunder IV and Thundaga III, as he seem to be weak to Thunder. However, he have to be carefull about not drawing hate, as Diabolos seem to always counter that by casting Sleepga II (it happened to us first two times atleast). He might also move too close to the mages too, so that they get slept once Nightmare goes off.
The BRD's job was to cast Ballad on the mages, while trying to stick Requiem and Slow (whatever that song is called...) on Diabolos. Lightning Threnody helps too, but really not needed. RDM enfeeble him (my enfeebles stuck 99% of the time, so he don't seem to be very magic resistant), as well as hasting the NIN and stuning/occasionally nuking to help him go down faster.
The WHM mainly concentrate on waking melees and tank up (first priority), aswell as curing anyone who needs it and removing their status effects.
The melees main job is to, offcourse, melee. Beat him without any deaths, easy as a pie.
Any set up is possible, you just have to think of your own one. Personally, I think he is easier that Fenrir, but that's just me. This was just our strategy, and we beat him flawless
