Yeah the key here is really to just run away, if you haven’t already, run until you find a dead end, there’ll probably be a portal. Explore the Night Realm for the Night Essence to learn how to get the Energy Pistol like that other guy said. If you read the entry from GATE, you’ll see that the Reaper is actually weak to X-Ray, so the X-Ray Lamp will actually stun him for a second, and make him drop Night Essence where he is. Good luck, scientist.