This page in our guide to Sekiro: Shadows Die Twice focuses on the fight with the Giant White Snake. You will encounter this boss on your way from Underbridge Valley to Ashina Castle Gate Fortress. From there, you will move to Ashina Castle. "Underbridge Valley" Sculptor's Idol is right after you defeat General Tenzen Yamauchi - the "bonfire" is below the destroyed bridge.
How to escape the Great White Snake?
Keep moving downwards by jumping from one ledge to another. After that, you need to jump over a chasm. Go through the cave located below. Crouch as soon as you reach the stone ledge with grass. The Snake will appear. Wait in the grass until the monster moves away.
Soon, you will reach more grass. Hide again. The Snake will lie on the grass, too. Approach its body - the Snake will roll, leaving only a small portion of grass.
Move from the grass to the stone ledge - do that when the Snake isn't looking. Hug the wall. When the monster turns around, run straight to the small wooden cabin.
Please note: You may notice an item that is guarded by a white snake. Never try to get it before you hit the eye. To pick up the item, hide in the hut (palanquin) and then hit the snake against the eye. As the snake goes mad with rage, run calmly for the item. Don't worry: the snake will not recover until you approach the final cave.
Don't get out when the Snake is looking at you. Press the attack button when its eye starts glowing red - you will stab the boss with your sword. Leave the cabin and run away. Get on the stone ledge and enter the cave - you are safe here.
Note - don't go into the fortress right after you defeat the boss. Unlock the Sculptor's Idol first - this one is located on the rock in front of the wall, on the right.