I think one of the most important points to remember when winning an argument with a theist is to first pin them down to their definition of god. If they only allow for some vague, could be anything really, kind of definition, then they can just twist around what you are saying with “But that’s not how I see god.”.
You need to be pretty focused on WHAT god is to the person you are arguing about, otherwise you will spend your time making claims that simply do not apply in this situation.
Is your god omnipotent? Omniscient? Omnipresent? Male? Female? Is is a vengeful god or a merciful god? Does your god provide justice?
Do any of the features of the argued god contradict each other? For example a god cannot be omnipotent and omniscient at the same time. They are mutually exclusive.
Of course at some point you may simply be told “God can do everything because he is god, even if it doesn’t make sense!”. When this occurs you have been given permission to walk away, because the theist has simply warped their perception so much that NOTHING can be used against god and god can do everything and nothing at the same time. If you are talking to a wall, walk away. You cannot help them anymore.