What was Big Willy Shakes going for?
Throughout the whole play, the Bastard speaks directly to the audience and interprets the other characters for us all the time. When he speaks, we listen.
It's also important to remember that he's really mischievous. But he's also King John's only real supporter at this point. And his advice for John is simple: don't back down. Fight as hard as you can. And one last thing: attack the way your enemies are attacking you.
With this little piece of advice, Shakespeare created a whole new phrase.