I'm Shard! He can live without a soul, meaning Reaper can just poke him. The only way for him to actually, be dead. Is for him to be completely shattered, than treated as if he was Nightmare. Just, make him non existent. He cannot feel pain, so even if he was melting he would never notice. He can't eat, or drink because he's hollow on the inside and the stuff he consumed would just get gross.