Depending on the Boss you are trying to defeat. They combine different types of damage to attack you, so I recommend trying to unlock all the Rosary Bead slots as soon as possible. You'll notice each Boss's predominant type of damage on your first few attempts; for example, Lesmes and her Infanta use Fire and Miasma Damage, Svsona uses Electric Damage, and so on. So, your task is to recognize their predominant type of damage and select a corresponding Rosary Bead.