After getting all achievements, I wanted to make a quick write up of the strategies I found effective for each boss.
Useful Tips to Beat Bosses
Warden of the Silent Sorrow
Look, beating this guy isn't rocket science. He only has 2 attacks, a leap and a slam, and both can be avoided with a quick dash behind him. Then, just wail on him until he dies.
When you fight 2 of them on the wall of holy prohibitions, the best strategy is to wait for them to both leap to you and then hit them with offensive prayers. Debla of the Lights and Taranto to my Sister will both do heavy damage and hit multiple times, and with drop of coagulated ink or smoking heart of incense will kill both very quickly.
Piedad can actually give you a lot of trouble on your first run, especially if he is the first boss you fight after the warden. The key here is that staying in close actually works a lot better than you might think. The only major time you need to run is when he faces towards you and does his branch area of effect attack. Otherwise, you can dash through his stomp then walk to avoid the branches, and parry his stooping arm sweep attack. You can also dash through his arm sweep, but it is far riskier unless you have little toe made of limestone to dodge further.
As well, note that Viridiana can assist with this fight.
In ng+ a solid strategy is to use Zarabanda of the Safe Haven in combination with drop of coagulated ink. Staying up close with this prayer active will do constant damage to Ten Piedad which you can double down on with you sword in openings after dodging or parrying.
First of all, if you fall into the fire at the bottom of the screen, you're done for. This means that you always need to pay attention to your positioning, and I like to climb first and attack second. Since there are 3 targets, getting into a position where you can hit multiple of them at once is a big help and will end the fight faster. You can parry some of their attacks, but since you will often be midair it is better to just try and avoid them. When the 3 merge together, avoiding the massive laser should be your first priority since it does a lot of damage to you.
Viridiana is also available to heal you during this fight.
In ng+ the strategy changes, but it is quite simple: Debla of the Lights. While you are using Debla, you do not take damage. This means that with drop of coagulated ink and heart of cerulean incense you can worry solely about positioning, and then fire off a Debla when the 3 targets are in a similar position. Notably, when the 3 enemies merge and do the massive laser attack, you want to actually stand right above them for this strategy. Simply use Debla, and you will i-frame the entire attack while dealing solid damage to the boss in return.
Our Lady of the Charged Visage
Rosary beads are a big help in this fight, especially as you are learning her attacks. Piece of a golden mask and other elemental defense beads will help keep you alive long enough to learn her various moves. There are 2 main ways to do this fight.
First of all, you can do the work early game to acquire Debla of the Lights. Even without prayer damage boosters, this powerful prayer will still do heavy damage to the boss while allowing you to avoid damage with its i-frames.
Secondly, you can go for the old fashioned sword approach. In this case, it is nice to have a damage boosting rosary bead like knot of hair or token of appreciation. This is because you will sometimes have few opportunities to attack the boss while it flies around and spams magic at you, so you need to make your openings count. Combining these damage boosts with the prayer Seguiriya to your Eyes like Stars will further increase your damage, and I recommend using the prayer when she comes to the bottom of the screen to maximize your damage.
Viridiana is able to assist you with this fight.
In ng+ this boss actually gets decently easier, because you have access to the empowered versions of elemental defense rosary beads. Make yourself a tank and stick with one of the same strategies for ng, and you should have no problem.
Esdras of the Anointed Legion
Esdras can be insanely difficult - until you realize that you can parry just about every offensive option he has. The exception to this is his lightning, so make sure you have the Perpetva's Protection rosary bead to help negate some of its damage. If you have used Viridianas help for the previous 3 bosses and possess Zarabanda of the Safe Haven, it is a solid choice here because it will constantly punish the boss even while you are parrying or dodging him. Otherwise, Seguiriya to your Eyes like Stars will help maximize you windows after parrying him or dashing behind him, because he is not always open to attack.
When he summons his sister at the end of the fight, I have found it best to just ignore her completely. She does not attack too often, and will sometimes miss you entirely without requiring a dodge. She dies when Esdras does, so simply aggroing him down will solve both problems at once.
In ng+ not much changes except for the numbers. Parrying is still far and away the best option, and if you have trouble with the timing you can use the Heart of Virtuous Pain to extend your parry window. Like against most bosses, Debla of the Lights is also very strong here, but you need to be careful that Esdras does not knock you out of the prayer before you enter i-frames and the pillar of light is summoned, so pick your windows carefully to use it.
Exposito, Scion of Abjuration
First of all: note that when the baby cries and a circle appears on the floor, you cannot afford to be in it when it closes. This is a grab attack, and this is an insta-kill grab attack at that. It is very slow to activate though, so just stay aware of the location of the circle and move accordingly.
Exposito has a bunch of different attacks, but all of them can be avoided with some standard dodging, or even just good positioning. The difficulty comes in with attempting to damage the moving target while you are almost perpetually under attack. In ng, you can take quite a few hits so you can usually afford to just attack all out as long as you have the healing reserves to top up a few times. If you use this tankish strategy, then rosary beads reducing toxic and fire damage are a good choice along with the standard defense ones.
Alternatively, this boss is very weak to being bursted down with prayers when it stops moving. Using Debla of the Lights or Taranto to My Sister with prayer damage boosters is an effective way to end the fight quickly, though be careful that the snake is actually staying still before you use them so that you do not waste your fervor.
Viridiana is available to help you here, and this is one of only two post bridge of the three cavalries fights where she can help you.
In ng+ things get a lot more tricky. With access to all prayers after a thorough ng run, I find either Tiento to your Thorned Hairs or Saeta Dolorosa to be very effective because they allow for a large margin of error while dodging the various attacks combined with sustained offense. Upgraded sword combos still do decent damage, and when the boss cannot punish you for them you do a lot of damage by combining these prayers with damage boosting rosary beads like lock of hair.
Melquiades, the Exhumed Archbishop
Big skeleton man looks super imposing at first, and can feel straight up impossible if you don't figure out the secret to damaging him. You'll need to cut away all of the hands supporting him to knock him down, and then attack his head while he is low in the arena.
Generally, an important note is that you can parry his staff strikes - which avoids knockdown and tends to push you away from the ensuing pillar of light.
The absolute best strategy here is using the sacred lunge attack. At this point you can have it upgraded almost to full, if not to full, and it does great damage while maintaining the protection of a dodge. As well, if positioned correctly, you can hit multiple supporting hands with one lunge - which makes this strategy both safe and efficient.
Once the head is down take your pick of offense. Both sword combos and prayers do quite well here and there is nothing he can do to stop you until the hands lift him back up, so have at it.
Along with Exposito, this is the only fight in the second half of the game where Viridiana can assist you, so take advantage of it.
In ng+ Melquiades can output a lot of damage, so I like to stack general defense rosary beads along with magic defense ones like consecrated amethyst. Offensive prayers are still fine here, but since the fight can be drawn out by constant movement by the boss I tend to favour survival ones such as Saeta Dolorosa and Tiento to your Thorned Hairs. These will keep you alive and you can rely on standard sword combos for decent damage when you down him. As well, the fourth and little toes of limestone rosary beads can combine to great effect here by massively increasing the amount of time you are in i-frames. Combine this with sacred lunge and you can output consistent damage while taking very little yourself.
Quirce, Returned by the Flames
Quirce is very fast and very erratic, so this is a fight where you might need to see it a few times to get his patterns down. You can parry a lot of his attacks, especially the charges and ceiling drops, and this will help minimize the damage you take. Quirce's scorched can be found in the same area as the boss, and will also be a big help as you learn his moveset. Offensive prayers are a good way to land a big punish when he stands still, since you will usually only have time for one sword combo anyways. Debla of the Lights is great here as it is everywhere, and can even get good damage on Quirce when he hangs on the ceiling to throw his sword like a boomerang. Finally, note that you can actually use the duck key to avoid a few of his left to right sword throws, which is easier than timing a jump or a parry.
In ng+ Quirce can become more of an endurance fight, so prayers like Saeta Dolorosa and Zarabanda of the Safe Haven are some solid defensive choices that allow you to maintain some offense at the same time. Quirce's scored bead is still strong, and you can actually stack it with piece of a golden mask for even more fire defense if you have the slots. The limestone toes are also nice here if you are having trouble dodging away from the flame pillars.
Crisanta of the Wrapped Agony
Consecrated Amethyst. Equip it, and maybe equip muted bell as well, because Crisanta does a ton of magic damage to you. A big tip is that when the boss enters the parry stance, you can actually attack into it, then parry her counter attack after she parries you. Parrying in general will make or break your experience with this boss similar to Esdras, so now is another good time to point out that Heart of Virtuous Pain can make this easier for you. As well, the sacred lunge is nice to close the distance, but be careful that she does not parry it because you might not come out of the animation in time to avoid the counter attack. During the final phase of dashing attacks, you can either parry her if you have the timing down or just jump over the attacks.
On ng+ Crisanta is very difficult due a combination of high damage output, high health, and a lot of mobility. Defensive prayers are a solid choice here, and I recommend having at least 3 quicksilvers used to upgrade your bile flask. This is because without the Pelican Effigy you are unlikely to be able to heal many times in a row without being hit, so you need to heal a lot when you do heal. Finally, frozen grape is actually a very useful rosary bead for this fight, since it will often be the difference between the boss finishing you off and you getting away alive - and the 2 magic defense beads are good in ng+ too.
His Holiness Escribar and the Last Son of the Miracle
First of all, His Holiness Escribar - or the light warmup to the Last Son of the Miracle. He can toss a bunch of varying magic attacks at you, but none of it is particularly threatening and he stands still for long periods of time. I like to abuse the sacred lunge here, because when fully upgraded it easily closes the distance to him and does massive damage on a clean hit. Ending this portion of the fight quickly and efficiently is key though, because you will want your resources for the second part of the fight and the large jump in difficulty.
In ng+ Escribar actually outputs a lot of damage, so I found myself using a lot of Tiento to your Thorned Hairs to make sure that I was not wasting too much of my healing. He does various elemental attacks, so you can also equip the various prayer beads strong against these - but since he does lightning, toxic, magic, and fire attacks, this requires a lot of rosary slots. His fireball attack is by far the most dangerous and it can and will kill you insanely quickly. Combining the limestone toes that allow you to dodge further and more often is a good way to avoid this attack, or you can make use of Tiento and just wait it out.
The Last Son of the Miracle is going to be either quite a difficult fight or a complete pushover depending on your strategy. Debla of the lights in conjunction with drop of coagulated ink and one of the prayer synergy mea cupla hearts will trivialize him completely. If you have been diligent with exploration and have a large fervour bar, a single time opening his mask will allow you to Debla spam him to death. This is a strategy that works equally well in ng+ and is extremely consistent.
Fighting him with melee is a ton more difficult, because you have to rely on the platforms moving in such a way that you can hit him while his face is vulnerable. Damage boosters like knot of hair and token of appreciation go a long way here because you need to make each and every opening count. In this case survival prayers like Tiento are a good choice so that you actually live to get hits in. This strategy gets even more difficult on ng+ and the best tip I have at that point is just to practice - because he has so many varying attacks and the fight goes on for long enough that you won't beat him without it.