ok you are m,aking the mistake in all assuming that this person has evrything you could ever attain up to this point...firslty what levels are your characters and what limit breaks/summons do you have....also what are you chars HP and do you have any HP Plus materia...what waepons have you got?

as before i would advise to train to lvl 35 roughly...do this in the room you fought the red dragon....you can also use this time to achieve more limit breaks...aeris has a good limti break called fury barnd which sets everyone to limit status...once you've achieve that..there no way you shoud lose....pummel it with limits bahamut, odin and attacks...it shouldnt last too long....

sit your ppl on the back row and they shoudl take less damage from his rush attack....