Where is this boss located?
(Senpou Temple, Temple Grounds) From the courtyard where there are several rat enemies, there’s an off-road path going downhill leading to an enclosed room where you can find several long-arm enemies.
However, if you want to be safe, you can ignore it for the meantime then continue up the main path until you reach the sculptor’s idol in the Main Hall. That way, you can just run past enemies and jump on the rooftops to quickly reach it as shown below.
How to defeat this boss?
Take time to eliminate the smaller long-arm enemies. There are two in the front, two in the wooden beams near the ceiling, and two more beside the boss. You can stealth kill these two from above but only one at a time.
Whenever you aggro the boss while taking out his minions, quickly grapple back up, leave the room and run towards the rock statues outside. This distance is enough for the boss and other enemies to lose aggro and reset to their original positions. Once it's alone, you can land a free stealth deathblow on it from above.
Its only perilous attack is a double upward swipe so either jump in front of it or away from it then repeat the continuous blocking of its attacks.
While fighting it, you have to press (not hold) your block button in rhythm to its multi-hit combo. Parrying its attacks will ensure that its posture gauge will take damage. Your own posture gauge will take damage too but you can quickly heal it by holding the block button from a distance, without blocking any attack. Keep blocking its attacks until its posture completely breaks. It doesn’t matter if its vitality is still high; as long as you break its posture, you can land the finishing deathblow.
* Yellow Gunpowder x2
* Prayer Bead
How to defeat the Long-arm Centipede Sen’un