Try playing defensively to learn Master Core's attack patterns. If Master Core isn't in his idle animation, try running away, jumping, and air dodging. Also, try equipment, most notably, the vampire equipment, if you have any.
Master Core goes through several phases, although on hardest, it's Human, Alligator/Scorpion thing, Sword, then your character's clone. The clone form is by far the easiest one. Well, actually, before you defeat Master Core, he becomes a sphere that you just pummel until it's destroyed.
I believe you're against the Sword form, correct? Well, again, if the Sword is ready to swing, keep your distance. The Sword often throws purple projectiles that act like boomerang. If the Sword is charging for a long time, then run and avoid the slashing. Finally, if the swords are spread out, stay in the bottom and try shielding/roll dodging if you're unfamiliar with how and when the swords throw stuff at you.
I've beaten Master Core on 9.0 difficulty with no continues. Trust my advice. ;P