Richter Strategy:
Carmilla is a formidable vampire. She floats around the room, summoning bats and throwing a scythe. Her most dangerous attack is when she transforms into a giant, floating mask that shoots blood projectiles. When she's in her human form, whip her relentlessly. When she transforms, focus on dodging the blood projectiles and attack her whenever there's an opening. The Holy Water sub-weapon is excellent for hitting her consistently, especially when she's low to the ground. The Cross can also be effective. This is a challenging fight, so conserve your hearts for sub-weapon usage. Stay in the center of the arena to give yourself room to dodge.
Maria Strategy:
Maria's agility and double jump are crucial here. Use her Doves to attack Carmilla in her human form. When she transforms into the mask, use your double jump to avoid the blood projectiles. The Dragon sub-weapon is very effective against the mask, as it can hit multiple times. Her Cat can also deal good damage if Carmilla stays low. Maria's Item Crash will deal massive damage to both forms. Focus on consistent damage and precise dodging.