Lol well can you convince someone of something they already know to be true? Both most recent Batmen are very aware they don't have powers. NolanMan needed a year of recovery and a robo-brace while BatFleck realized it from his multiple injuries after fights with regular guys and metas alike.
As for Joker I don't think anyone can convince him of anything he hasn't decided on his own.
That doesn't change anything at all. They still fail. And they apparently
Spoiler:
upped Dunns water weakness by a lot just so this plot could happen. It seemed in Unbreakable he couldn't swim because he was so dense and heavy. But then water just suddenly weakens him to the point that someone can step on his head in a puddle and he can do nothing about it.