Playing against the AI using a custom deck

I've figured out all the deck building, and so on, but I've become bored with facing the same old strange/bad decks from the AI in challenge and campaign mode in 2014 by now.
So how do I battle the AI that's wielding one of my own creations? I've seem to read about people doing that, but I've not found how it's done. Maybe I'm just overlooking something obvious?
So how do I battle the AI that's wielding one of my own creations? I've seem to read about people doing that, but I've not found how it's done. Maybe I'm just overlooking something obvious?