* Asterisks denote story required completion.
The digging mini-game has you escort five different people down into the depths of the earth for a specific target. You will have to employ several different Brush Techniques and strategies in order to make it down to the very bottom.
Strategy #1: Use Power Slash to turn your comrade around. This will prevent them from walking into spiked blocks or going over an edge you don't want them to drop down from.
Strategy #2: Use Galestorm (gained from Gale Shrine in Kusa Village) to blow your comrade over one block and across a gap. This is useful if there is a spiked block in the way and, while you can jump over it, your comrade isn't as smart.
Strategy #3: You can use Cherry Bomb (gained from Pyrotechnist's Hut in Shinshu Field) to explode several blocks at once. This is especially useful when you're completing the fifth dig.
Strategy #4: Use Power Slash to cut through blocks that have a crack through the middle. They won't break any other way.
Strategy #5: Light brown blocks break with one headbutt or one dig, while the darker brown will require two hits.
Strategy #6: After the first dig, you will find that there are some water wells in later ones. You can use Waterspout to push your comrade up if need be, or let them simply swim to the other side.
1*) Sasa Sanctuary - The springs of the Sparrow Inn are all dried up, and it is up to you and Mr. Bamboo to recover the precious water that lies below. The earliest of the digging mini-game, it is also the easiest. You will have plenty of time to spare, so take care of your comrade and just learn how to play the mini-game efficiently.
2) Taka Pass - After you acquire the Galestorm technique from the Gale Shrine, you can help Bingo the treasure hunter out. This guy will be near the exit to the City Checkpoint, off the beaten path and observing a pile of leaves.
3*) Sei'an City (Commoner's Quarters) - When you enter Sei'an City, you'll find that it is desolate and being plagued by some terrible disease coming from further within the city. To actually go further in, you must restore the water that runs through the city to appease Benkei. The dig mini-game is located in the northwest part of the city, which would be more easily accessible for you if you followed the canal towards it.
4*) Sasa Sanctuary - Within the second arc of the game, you will need to return to Sasa Sanctuary to aid Kaguya out with exploring the royal crest in the back of the grove.
5) Dragon Palace - I'm not quite sure if this is "necessary", but it will help you out in the end. Once you acquire the Shell Amulet required to enter the Water Dragon's Den, you can instead go down the other hallway on the first floor and come across the Dragon Palace Springs. Five maidens are attempting to restore the spring, and the largest one will actually enlist your aid once you start the process off for them. This will actually reward you with the secret brush technique to add on to Waterspout, allowing you to use the Mermaid Pools without needing coins.