This is one battle you can easily think it is not a boss fight at all. After all, you have a large number of mobs who just turn to look at you and occasionally fire a spell or two.
However, things can get a little hairy quickly enough. Especially if you stay away for too long and the Deacons start casting a fog spell. Should you notice something like that, kill at least one of the Deacons to break the spell. Apart from that, this is a pretty straightforward fight.
1. Charging the crowd
The Deacons are weak against physical damage and you will be able to kill them with little worry. Getting a weapon with a larger sweep range is going to make it even easier.
The idea here is to follow a red aura which will go over an enemy. In the first phase of the battle, just charge in and keep an aggressive strategy against the corpse with the red aura.
The second phase becomes only slightly different as now there will be a couple of new Deacon types, and the aura will stay fixed on a single Deacon for the rest of the battle. No matter, the idea remains the same. Circle around, find an opening, charge the Deacon with the aura while killing some of the rest, roll out, circle and repeat.
Overall the Deacons deal little damage and the only thing you will need to look out for the black orb attack in the second phase, since it can bypass obstacles.
2. Using magic (Try not to)
The main problem with using spells is that the Deacons keep respawning and fast enough. It gets even worse in the second phase of the fight, since all of the other Deacons will try to shield the one with the red aura as much as they can.
Long story short, you will either have to be skillful and lucky to win this fight with a single target spell. Now if you have AoE spells like Firestorm or Soul Spear, then you can give it a go. Just make sure you aim as best as you can so that the damage you make is not wasted on respawns. Oh, and they have resistance against magic as well, so there goes that plan.
Unless you have Boulder Heave on your pyromancer. It deals in physical damage, it is AoE and ranged.
In the end, this is going to be of an annoying than a hard battle. If you have the Hollowslayer Greatsword this is going to be a breeze. And you can also use Alluring Skulls or Rapport for some extra help.