Richter Strategy:
Dullahan is a headless knight on horseback. He attacks with a lance and summons fireballs. He's quite agile for his size. The key is to attack him when he's on the ground and dodge his charges. When he jumps, he's vulnerable to attacks from below. The Axe sub-weapon can be effective, hitting him as he rides by. The Cross is also good for consistent damage, especially if you can get it to hit him twice. Be mindful of the fireballs he summons, as they can track you. Stay mobile and use your backflip to avoid his charges. His head is his weak point, but it's often out of reach.
Maria Strategy:
Maria's double jump allows her to easily clear Dullahan's charges and lance attacks. Her Doves are excellent for consistent damage. The Dragon sub-weapon is very effective, as it can hit him even when he's riding or jumping. Use her speed to get in close, unleash a Dragon, and then retreat. Her Item Crash is devastating. Focus on dodging the fireballs and continuously attacking with your Doves or Dragon.