Notice there is a slight pause before it attacks. Evade the attack, then attack it with your knife.

Run up to the Madcap and immediately dodge. This triggers the enemy to attack, and you will have already dodged it. Once his fork is stuck in the ground, run up and hack away.

The Hobby Horse works well, as it breaks enemy shields. Use it against the Madcap with the shield, Cannon Crab, and other similar enemies.