Because nothing is easy, Father Ariandel will get furious at seeing Sister Friede fall. He will bring her back to life with new and improved abilities. He will also join the battle. The good thing is that they share a health bar. So, it does not matter who you attack as the only thing that matter is the health bar going down. But the bad thing is that you need to pay attention to both attackers. This is because each can deal a great amount of damage.
Father Ariandel has a powerful attack. He goes between holding the bowl high before spewing a large blast of breath that will move from side to side. Then start yelling and crawling towards you bashing the bowl down with length. Before he stops, he will go forward few times. He will either continue smashing the bowl down or if you attack him for longer period od time, he can take a large swipe with the bowl. Then he will douse himself with the bowl’s content to spread fire everywhere and then explode in a huge fireball.
Sister Friede will swipe her scythe to send a long wedge of frostbite toward you, which makes it harder to defeat them. When this frostbite explodes it raises your frostbite meter. It also deals massive damage. This makes it hard to defeat Father Ariandel. Between her and Father Ariandel, you need to move at all times. If you get close enough, Sister Friede will re-use her slash attacks up close and may end with a powerful attack. But she will not be using her invisible high-damage attack. She will turn invisible when she needs to build up a large heal for both of them. If you see her disappear, circle the arena until you see a large glow of golden light- and race forward to attack her at all costs, or she will undo all the damage you have been dealing and you have to start all over again to defeat them.
Focus only on one!
Between the Sister and Father Ariandel, it is your choice on who you focus your attacks. Father Ariandel is usually open to attack after he goes across the arena. And Friede has the one easy to avoid frost attack from a distance. But Friede is equally open to counter attack when healing and after her slashes – like she knows where Father Ariandel is and what his next move is. If he is closing in with his slamming attack or prepare to breathe fire, this is the time to keep moving. But if he is dousing a fire on himself away, you will have time to engage Friede directly.
This is a tough phase because you need to deal with many targets just to defeat only one. But you must learn it and practice in order to defeat both of them and have Estus to spare. The important this is that you need to manage your frostbite. You will take a lot of frost because of her attacks, but if you get distance on them you should be able to consume Rime-blue Moss Clumps to get rid of frostbite. Executing them both will give a one-time drop from Father Ariandel giving you a Titanite Slab, but then you will meet the final Boss, Blackflame Friede.
This is the second stage where you have to defeat Father Ariandel. It does look more complicated, but wait until you see how you can defeat Blackflame Friede.