I agree that a save point between phases would make the game easier, but there is an appeal in beating all three phases in one gauntlet. Maybe it can be part of a difficulty setting that could be implemented in the future.

As for the fight itself, I will agree it's the hardest part of the game. Here are some tips that helped me get past phase 1 and 2 of the fight:

1) That white corpse hanging at the top of the screen is not tied to the wall - You can go behind it. Knowing this vastly increases your mobility.

2) Gary instantly destroys seals when he walks over them. This is especially noticeable after his multi-lunge attack, which tends to clear most seals from the floor.

3) When Gary does his multi-lunge attack, hide up in the corner. He usually moves side to side, with a small amount of verticality, and rarely gets deep enough into a corner to hit you. Just be careful he's not using his single, aimed lunge, instead.

4) Collecting more notes supposedly makes boss fights easier, in that you do damage faster. I can personally attest that this is the case in Chapter 1, but cannot confirm it for Chapters 2 and 3. Worth a shot, to hedge your bets, though.

5) John's hitbox is only about from his waist to his ankles, and the seals' collision box is smaller than it seems. If you get stuck, you might be able to walk through the top of a seal, rather than have to walk down and around it. Knowing your hitbox can also help you avoid Malphas's fireballs.

6) If you're on the same side of the room as Malphas during Gary's multi-lunge attack, waggle your cross at him, instead. They share a health pool. If you're lucky enough to catch them both at the same time, you'll deal double damage.

7) Try to only attack Gary from the sides, when you can. His hitbox is hard to determine when he is in front of or behind you, much like in Tiffany's boss fight.

8) Try to only move up and around Gary right after Malphas casts his fireballs, so you have maximum time to maneuver before he casts them again.

9) When Malphas does his stomp attack, he'll stikk kill you even if you're above the spot he's targeting. You can't move behind him when he's trying to land on you. It doesn't make a ton of sense, perspective-wise, but it's good to know ahead of time.

10) Try to stay away from the braziers. For obvious reasons.

11) Try not to hug the walls too closely, so that if a seal spawns right up against it, you won't have to course-correct as much.

12) When you get to the part you're having the most trouble with, take one or two lives to just practice avoiding enemy attacks without worrying so much about attacking. The best offense is a good defense, after all. There's no time limit, so if you prioritize staying alive, you'll eventually be able to whittle the bosses down.

13) If you get really frustrated, take a break and go hunt for unfound notes in other chapters. Frustration will 100% kill your abilities, and exorcising some lesser demons like the Spindly Lady or the Graveyard Demons will help reaffirm your abilities as a competent priest...And maybe strengthen your faith, besides.