How do you design "Custom AI3"'s and "Custom False Gods"? I've seen screenshots of those, but I can't find out how to make one myself.
Use the
Elements trainer tool, which Zanz has refused to link anywhere people might find it on their own. Above the AI0 button is a red button labeled "import". Enter the deck code for the deck you want to use (which can include more than copies of a single card), and tell it whether to have it played by AI3 or AI6 (False God), then hit import. Once you do this, the regular ai3 and false god buttons will cause you to face this deck as well.
EDIT: Pic to show what I'm talking about.