Great Colored Carp achievement in Sekiro: Shadows Die Twice

This is an extremely complicated achievement but I will do my best to include everything.

TLDR
1: Reach Fountainhead
2: Kill Lightning shooter atop the tree by the Great Sakura idol.
3: Grapple up to Merchant in a pot.
4: Purchase Mask Fragment for 12 Treasure Carp. (possibly own all 3 mask fragments).
5: Talk to him again to acquire "Truly Precious Bait".
6: Swim underwater through the cave, past the Great Carp, to Palace Grounds idol.
7: Grapple hook onto the roof, then tree, then left to reach Feeding Grounds.
8: Ring bell, feed him Truly Precious Bait.
9: Fast travel to Guardian Ape's Watering Hole. Collect Whiskers item.
10: Profit.

Long version: First off, find the Great Sakura idol and kill the enemy on the nearby tree who shoots lightning at you in the lake so that you can now safely swim around.

Then, from the third idol in fountainhead, Flower Viewing Stage, make your way up and toward the back of the area to a closed double door and a waterfall. Facing the door, there should be a waterfall to your left. Cross over this waterfall onto a grassy area with a tree. Looking at the tree you should see a Grapplehook mark on the rocks behind it and to the left a bit. You may have to move around some if it doesn't appear right away. Grapple up and go through to the next area where you will find the Near Pot Noble idol.

Next to the idol is a man in a pot. Talk to him and you can purchase the third Mask fragment. (Note: I already had the other two pieces in my possession, thus completing the mask. I am not positive if this is required or if simply buying the piece he is selling is enough.) Upon buying the mask fragment, talk to the npc and exhaust his dialogue. He will offer you a Truly Precious Bait to feed the great carp, which will kill him. Normal "precious bait" can be found elsewhere and only feeds him, it will not kill him. Only this "truly precious bait" will do the trick.

The Truly Precious Bait cost 12 Treasure Carp Scales. These can be found in any underwater area by killing the pink dolphin-looking fish. There are many in this area, some are also on the lake bottom appearing as items.

Now, as far as getting to the Feeding Grounds area, you must swim through an underwater cave in order to first reach the Palace Grounds idol. Start off at the Vermilion Bridge idol and step out to where you are overlooking the whole area. If you look in the middle of the lake, on the right hand side, there is a hut floating/sticking out of the lake. This is what you should aim for. When you reach the hut turn right and dive underwater. Eventually you should see the underwater cave on your left. Swim through, avoid the great carp, emerge on the other side to the Palace Grounds idol.

From here, enter the doors, turn right and go up the stairs, you will pass a chest on your right. Once outside again, immediately turn around to your right. You should see a grapplehook icon on the next building. Get up onto the roof and go to the far edge. Look up and to your left to see a tree branch sticking out. Grapple to this branch (running start advised) and then down to the rooftop below/left. Continue down in this direction to find the Feeding Grounds idol. You can speak to the man here, he will tell you to ring the bell to feed the great carp.

Ring the bell, select the Truly Precious Bait, Travel to Guardian Ape's Watering Hole.

I know this was very long and confusing. I will update as needed.

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