if you are referring to the final boss ie the giant head and the tentacles than there are numerous ways to defeat him it mainly depends upon your party. a good strategy is to use the tenteikura skill this ups a groups evasion for 3 turns then just wail on him and have whoever is using tenteikura close by to recast it as needed. Another extremely nasty trick is to bring Mayuri to Hueco Mundo and summon Konjiki Ashisogi Jizou. Jizous venom when he is summoned next to suzunami but away from your team will dish out about 1,000 points of damage per turn. additionally u can augment konjikis survivability with tenteikura and a healer nearby Nemu is a great choice as his venom will not affect her or mayuri if all ur packing is healing items.