The key here for me is to only equip the bare essentials of materia, my friend asked me to do this part for him because he had only leveled up to about 25 an so he was struggling, I gave Aeris Cure-all, Cloud- Deathblow and Barret-bahumut, if you can pick up barrier from rocket town give that to either Aeris or Barret, also most of Demon gates attacks fill your limit bar up straight away, so they key to this battle is to just have enough hp to sustain his major attacks, then quickly use cure-all or get Aeris to use healing wind and just use your other characters to chop away at his hp wit summons and limit breaks.