How do I kill the Abyss Watchers?

You can try summoning help! Use Ember item then summon either players willing to help (online play required) or NPCs - for this fight you can summon these folks:

• You can summon Sirris of the Sunless Realms after speaking to her for the second time in the Firelink Shrine. Her summon sign appears to the left of the entrance to the boss room.
• You can summon Londor Pale Shade if you have 5 dark sigils from Yoel of Londor and are on good terms with Yuria of Londor.
• You can summon Black Hand Gotthard.
• You can summon Yellowfinger Heysel if you have defeated her invasion and given 1 Pale Tongue to Rosaria.

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Phase 1.

The key factor here is the "in-fighting" mechanic that this boss has.

Initially, it'll be 1v1. Then, (after 20 seconds) a bell will toll, and another one will spawn. This will make the fight 2v1.

A couple of things to remember here:

• Each one has their own Health Bar. You need to keep an eye on which is the "Primary" enemy with the on-screen Health Bar.
• He is hostile to the Player (and summoned allies).

After another 20 seconds another bell will toll and a third one will spawn. This one is unaligned (with red eyes - to make him identifiable), so be careful not to aggravate it. If you are the closest to it, it will attack you, but it will aggro to the last enemy that hit it.

Once you kill one of these two, they will respawn every 20 seconds.

Tactics:

• Summon friends (as arghtype suggested) to spread out the enemies, allowing you to focus on individuals more easily.
• Try and use the red-eyed Watcher as an ally.
• They are weak to Lightning Damage.
• With practice, you can learn to parry their attacks, and with friends, it makes them easy to distract and get in back-stabs (as per Jutschge's answer).

Phase 2.

Once you have killed the Primary Watcher, all three will drop dead. This activates Phase 2. The upside; there is only ever one to fight. The downside; bigger attacks with fire damage.

Most of the tactics from phase one still work here; gank tactics, parrying, and lightning damage. The problem is that the fire damage from the effects can cause you to get hit and lose your chance to riposte or back-stab - even if it simply clips you.

Tactics:

• Fire reduction equipment; shields and armour, etc.
• If summoning help, he will aggro to the last person that attacked him, allowing you to create openings to heal and back off, as necessary.
• Use the "5 D's of Dodgeball"; the i-frames will help you move through/avoid the fire effects, and avoid getting clipped to interrupt your attacks.

On a side note: you can retrieve the Lightning Spear miracle from the tower/bridge where they Stray Demon is patrolling (around behind the gate of this section). This can help with a steady source of lightning damage, if you run out of resin packs.