Once you’ve taken the boss’s health down to zero, don’t celebrate just yet; the second phase of the fight is about to start. After a sweet cutscene where the main boss absorbs the essence of all the fallen Abyss Watchers, it’s time to get serious. Unless you’ve got a good fire and physical resist shield, I suggest putting away your shield and two-handing your weapon instead; blocking with the shield isn’t going to do much for you unless it has high fire defense. Instead, rely on your rolls and i-frames to dodge the attacks.
In this phase of the fight, the Watcher’s sword gains fire abilities, and he’ll perform thrusting dashes towards you if you’re far away. Take note that this will also leave a trail of fire in his wake, so be careful not to get caught in them. His attack combos will also unleash short bursts of fire around him, so make sure to dodge away and to the side, and wait for the fire to dissipate before attacking. Your best window for attacking will come when the Watcher leaps up and slams his sword into the ground. This will leave him open for just a second, and is also a great chance for getting backstabs.
The key to winning this fight is to just be patient and not get greedy with your hits in the second phase. The hardest part of the fight is being able to dodge roll successfully without getting hit, so make sure to time your dodges just as the attacks are about to land. Getting caught in one of his devastating combos will put you very near death.
