Plus your argument hinges on thinking that a being that does not do some of the things that God does in the bible is greater than one that does. Part of the belief in the God of Abraham is that whatever he does it is the greatest action, simple because he is God. That's all part of religious circular thinking.
In which case you ask them the following: If God commanded you to rape your mother, brother, and daughter, would you do it?
If they answer "no" then they don't believe their own argument.
If they anwer "yes" then you beat them into submission with their gross immorality, that any sane person observing the discussion would see as abhorrent.
Their best answer is not an answer at all, but to claim "God wouldn't command that". Then you ask them how they know that? Do they know the mind of God? Really??? How?
They're in a tough-to-losing position already. And you also point out that they can't argue that God wouldn't command you to rape family members because it is immoral, since God commanded a man to kill his own son, commanded innocent children be slaughtered (Amalekites), commanded that children be killed just for striking their parent, condoned slavery, and so on -- these too are far outside the character of a morally perfect being, but God did them - so God has "already gone there", so to speak. There's precedence for that kind of thing. So if we cannot know beforehand that those "seemingly" immoral things were "actually" good, then we can't know beforehand that God commanding you to rape your children and parents is not "actually" good.
And yes, I realize the above does not end the discussion in victory in the eyes of obstinate believers. But it is a strong line of reasoning against their claims.