Harry should Dark Hallow demon reach. With the islands defenses, it would be hard for anyone to interfere. With the islands remoteness, it is likely no one would be hurt, and Harry could just put a circle around the island anyhow. A dark hallow on demon reach would suck up all the power of all the bad nasties on the island. So in one stroke, Harry becomes more powerful and both Fae Queens and the Erlking put together, and he deletes the demon reach prison forever.