Listen. It's really easy if you have learned the move "Regulate" and "Restore".
Becuz u can then just keep using regulate on him.....which drains 10 focus. after Baron's focus is out.....he will use a move which restores focus in the cost of his life. just keep doing this......restore health if you have to......and you're done. And after Baron's life reaches like 4000......then you can just beat him using attacks.