Like all of the bosses in Dark Souls 3 that rely on superior numbers, the Deacons of the Deep are reviled for being a cluster of weak cowards who are more tedious than actually difficult. However, they can still pose a threat as they have many chances to land a lucky hit with either their annoying little jabs or their fireballs.
Furthermore, the true leader of these hollowed heretics doesn't even reveal himself until midway through the boss battle, meaning that there isn't a quick or easy way to end this fight without having to suffer through waves of respawning wretches. Thankfully, there are some ways to take out one's aggression on the clerics, and the archdeacon himself, in a manner that will result in a decisive victory.
Melee Strategy
The Deacons of the Deep have quite a few resistances, including magic and dark damage as well as poison and bleeding. However, they are weak to physical attacks, which makes melee arguably the best strategy to face them. In addition, they are hollowed, therefore weapons that deal bonus damage against such foes are ideal to use in this boss fight. A great choice would be the Hollowslayer Greatsword, which can be created from the Soul of the Rotted Greatwood.
This greatsword's 20% bonus damage against hollows can let the player cleave through the horde of hideous clergymen efficiently, especially when combined with the twohanded light attacks that unleash very wide horizontal slashes that can hit many enemies at once. This strategy can be enhanced by equipping the Pontiff's Left Eye ring, which can be transposed from the Soul of Vordt of the Boreal Valley. The healing gained from successive attacks can totally negate the minor hits that the deacons land on the player, allowing one to brute-force this fight with ease. Just be sure to take out the 4 blue-robed enemies that show up when archdeacon Royce appears, as they can heal him as well as unleash a curse effect that will instantly kill the player.
Ranged Strategy
Most bows are not advised for the Deacons of the Deep fight, as accuracy and timing are required for such ranged weapons, though the deacons will not be likely to allow the player either. However, Greatbows can be used if one positions themselves as far away as possible within the boss room. The flimsy priests will not be able to stand up to greatarrows of any kind and will get knocked down or outright killed by shots fired from the player.
The downside is that Greatbows are quite cumbersome and slow, specifically since they usually need to the spiked into place before firing, which leaves the Ashen One open to counterattacks, particularly the deacons' fireballs or the great dark orb launched by Royce and his cronies. For this reason, the Onislayer Greatbow is the best to use here, as it is the only one that does not need to be spiked into place to be fired, allowing for enough mobility to sidestep incoming attacks while loosing greatarrows into a crowd of hollowed foes.
Caster Strategy
Fire and lightning damage are not technically a weakness for the deacons, though they are not resistant to these damage types either, making pyromancer and miracle builds fairly well equipped to take out this boss. Since the hollowed priests also use spells, the battle could turn into a shootout of sorts, though with fireballs and bolts of lightning flying around instead of bullets.
It is recommended to use longer-range spells as opposed to close-quarter ones, as the crowd of enemies in this boss room can quite easily interrupt one's casting with candlestick pokes or other annoying little attacks. For pyromancers, Fireball or other variants are a solid choice for their relatively short casting time and high damage. Any that leave behind lava is especially helpful as the dull-witted deacons will just stand in the burning liquid and die.
Miracle builds will have a bit of a harder time due to the lack of offensive miracles until one reaches the late-game. However, anything that unleashes lightning damage can be a great boon. Since caster builds are a fair bit more squishy, it is likely that players with this playstyle will need to heal. Utilize the corners of Aldrich's massive sarcophagus to do so, as it provides more cover than the columns that ring the area. Just be careful of the large black orb that Royce and his healing quartet can launch, as this can go through objects. Thankfully, it has no homing and the projectile flies fairly slowly, so avoiding it should not be too much trouble.
Area Of Effect Strategy
Foes that cluster together are just asking to get slammed with an AOE attack. Fortunately, there are quite a few ways to unleash such assaults. The weapon skills of a few weapons can be very helpful in this regard. The Demon's Greataxe is a prime example, as its weapon skill, Demonic Flare, lets loose a huge fiery explosion that can decimate the deacons' numbers. Lining one of these up on Royce as he spawns in with his blue-robe buddies can be immensely satisfying and devastating in equal parts.
The Falling Bolt weapon skill that belongs to the Dragonslayer Greataxe is another great AOE attack that performs similarly to the Demon's Greataxe's Demonic Flare, though with lightning damage instead of fire. On a related note, there are various pyromancies and miracles that also yield splendid results.
Despite taking longer to cast, it can be both fun and efficient, if risky, to summon a big pyromancy like Firestorm or Chaos Storm to immolate the adversaries. The latter is recommended over the former since it leaves behind some lava that the dull deacons cannot seem to avoid. The Wrath of the Gods miracle is probably the best Faith-based spell to use here as it not only is an AOE attack but also deals physical damage, which the deacons are weak against. There is also no denying that it is tremendously cathartic to watch the clergymen go flying.