Which part of the battle are you stuck on? is it the beginning of the fight where you are on foot? or when you are flying around in the air? If it's the latter, just fly around, use the attack button to hit leviathan and stay out of his attack range. If it's the first part of the fight, you'll want to hit the square button to guard when he attacks you, since there isn't much you can do, you can attempt to warp strike him with the triangle button but that sequence shouldn't last too long as it is scripted and the story takes over from that point.