How do you kill the Great Colored Carp in Sekiro: Shadows Die Twice? This optional encounter isn't strictly a boss fight, but defeating it does unlock a PS4 trophy for your collection. Found in the Fountainhead Palace area towards the end of the game, you won't be fighting the Great Colored Carp, and will instead need to use another tactic to bring it down.
Sekiro: Shadows Die Twice - How to beat the Great Colored Carp
After the Palace Grounds Sculptor's Idol, turn right in the next building and back outside. Grapple up to the roof, and then across a tree branch and another temple roof. You should find a new Sculptor's Idol called Feeding Grounds. Speak to the NPC there and ring the bell to make the Great Colored Carp appear.
In order to kill it, you need to feed it Truly Precious Bait. You have two opportunities to get this item, from either of the Pot Nobles (the NPCs that live in blue pots). One is in Hirata Estate and the other is in Fountainhead Palace. After giving one of them enough Treasure Carp Scales, speak to them and they'll give you Truly Precious Bait. Simply return to the Feeding Grounds idol and feed the Great Colored Carp the bait. This will kill it.
However, you're not quite done. Travel to Sunken Valley - Guardian Ape’s Watering Hole. Here, you'll find the dead body of the Great Colored Carp. Approach it and collect the item -- Great White Whisker. This will trigger the PS4 trophy. Also, bring the item back to the Pot Noble who gave you the Truly Precious Bait, and he'll turn into a carp himself!