The main reason that your set was rejected was mainly the item choice. LO is a bad item for all-out attacking Cobalion, because very often he does more damage to himself than to the opponent, cutting dow his lifespan without doing anything significant. But Expert Belt gives almost the same firepower, still ohkoing Blissey after SR, whith no recoil at all, which means that if you play Cobalion right, he will have enough life left to take on the physical threats it is supposed to.