This enemy is strong but you are best only using one summon in the shape of Bahamut. The best way to cause damage to this monster is with physical attacks and limit breaks, so use Bahamut and then go on the offensive physically just make sure to keep healing and use haste and barrier if you have them. Also if you remove the magic materia and other summons it will raise your HP too so you could also try that too but it is up to you.