http://ffxiv.gamerescape.com/wiki/A_Feast_of_Fools

First of all, like Ifrit, you need to kill some NM's. They are all listed in the above link, please read it. Secondly, you need to know that they all must die quickly before the big fat one comes… or we die! they will all revive when he pops up and dance about, if they are all dead then we should live. Third part. as i've seen it someone kites the king or keeps him away from the group and we kill each moogle one by one.. The MRD has a nasty aoe so keep him away aswell. It's best to take out the mages and the bard first and it seems leave to glad to last.

Once again my friend as WHM. Watch this even if not WHM.
http://www.youtube.com/watch?feature=player_embedded&v=NTomk8ZHH3g

http://xivpads.com/?-Guides&Good_King_Moggle_Mog_XII_Battle_Strategy
Read this aswell.