One thing I love about christianity in a funny haha mock you kind of way, is that they still believe in, in the words of george carlin, "An invisible man in the clouds", no matter how many times you prove he cannot possibly exist.
A good one of mine is to point out to them that god is not immortal, in fact he must be eithe very mortal or very non existent.
Like so:
To manipulate anything in this universe, you must interact with some form or energy or substance, thus beng made of a similar energy or substance in this universe. Therefore have something that is poisonous to you or react badly with your own substance. Thus mortality.
Also, with that same opening reasoning, you must therefore be able to be affected by some form of energy or substance in this universe, thus we could harness it, weaponise it and kill god.
EDIT: Incase it went over some believers heads, if he is unnfected y anything in this universe then it is impossible for him to affect anything in this universe either. The two go hand in hand, if such a paradox did exist it would rip a hole in the fabric of space and possibly even time too.