Deacons of the Deep is one of the easiest boss fights in Dark Souls 3. The first time you fight them, you may be confused about how to damage them, but you will figure it out rather quickly. The trick here is killing the Deacon that is glowing red. You will deal no damage otherwise and the Deacons respawn infinitely so there isn’t much point in killing them. The strategy for this fight is very aggressive. You simply want to charge in and kill the Deacon glowing red as fast as you can, noting which Deacon the red light moves to next as you retreat and gather your stamina.
Keep repeating this until phase 2. As the Deacons deal very little damage don’t be too worried about getting hit, just heal whenever you need to. In phase 2 the Archdeacon will appear and the deacons will gather around him to protect him. Having a long weapon is more effective here, as it will allow you to hit the Deacon through his bodyguards, but it can still be done with a shorter weapon like I used. You want to try to find an opening and run in and exhaust your stamina hitting the Archdeacon and as many enemies around him as you can at the same time (if your weapon can). Although these Deacons respawn if killed, it will take a second, leaving your next assault on the Archdeacon much more open and making it a bit easier to get in close.
As found on Youtube