How to easily defeat any argument for God

[BrianSoddingBoru4]

Though your interlocutors here continue to think of God decreeing the Good like laws, theologians would say that God is the sum total of all the good aspects of all the things in the world (plus infinity). This is what Dante says. So you are correct, I think, to say that God couldn't reverse goodness and declare by fiat that the Holocaust was actually good. Because there is no possible world in which genocide encourages human thriving.

If this is a description of God, then goodness exists independent of God and we don't need God to be good.

As to the Holocaust, I can think of situations where genocide encourages human thriving, but that's not your point (at least, I hope it isn't).  If God can't declare by fiat that the Holocaust was a good thing, then there is no point whatsoever to God as either a source of or a conduit for moral behaviour: actions are good or bad irrespective of the what God has to say about it or God's nature.

Boru