First, on the God-is-an-alien thing, I don’t see how that can possibly be a useful avenue of debate. In one sense, God is very alien, very other from us. In another sense, he is what (who) we are made to be wholly united with.
As to “stumping” an atheist, I think the best approach is to go after his assumptions (many of which he will not even realize exist). For example, if there is no God, there is no good or evil, there is only what we have been conditioned by evolution to do or not do, which has no moral component. It’s claimed that theism has a problem of evil, but it is really atheism that has a problem of evil, or a problem of lack of evil.
Another fundamental problem with atheism is accounting for anything - that is, why is there anything rather than nothing. They will bring up all sorts of hypotheses such as eternal universes (almost certainly not possible) or multiverses (no evidence, and by definition there cannot be any evidence, and still has the eternal universe problems).
Check out Edward Feser and William Lane Craig and Frank Turek for lots of good arguments and debates. The latter two also have lots of audio and video material.
Edward Feser states (and argues) that atheism is “deeply immoral and irrational”. This is exactly the opposite of how atheists view themselves, and you can really rock them back on their heels by using arguments to show that if atheism is true there cannot be either morality or rationality.
