I found that the easiest and quickest way was to attack Brittany off the start. If you defeat the enemy army and are lucky, you'll capture their king allowing you to win the war quick.
Next you will need to secure 2 duchies. Brittany is it's own duchy so that takes care of one. For the other, You can either try Cornwall if they aren't allied with East Francia or take a few counties in Ireland and buy the duchy title afterwards.
After that, you'll just need enough gold to create the Kingdom of Brittany title and you're done!