hack away at him with meteos (or if you dont have that powerful magic and summons.)
and hell bite the dust sooner or later.make sure your spells hit all of them and constantly have someone cure with white magic.have fighters focus on a single part of X-death because when ones dead just go on to the next and make things easier.have one of your fighters occaisionally use an ether/elixer to get back mp.once one parts dead it gets easier and if you want you can make 4 time mages with white magic if your health is high enough or have 4 of the job class with the most health 3 know time magic and meteo and 1 knows good white magic but its all about killing him of quickly so he has less time to damage you.