I am a big Demogorge fan, but it seems that his powers might be geared toward a different type of opponent. He is geared toward magic-based, deity-type, mystic beings.
Much like the way Drax has the "silver bullet" for Thanos, Atum may be specifically formed and focused to harm "Mystics".
I'm afraid that Tyrant, whose power and reality is firmly based in the "real" universe he exists in, could be too much for Demo.
