Undiggable area even though I have proper skill

OK, one vid is worth 1000 words:

This is in Kamui.

• Cherry Bomb did not have any effect either.

• I do know it's the entrance to the third and final devil gate.

• I do have the Digging Champ dojo technique.

• I was able to dig the other two devil gates just fine.

I am well aware that in the above link it says:

The Devil Gate Trials will be open after defeating Lechku & Nechku in the Wawku Shrine

Indeed, I still did not reach the shrine. However, further search gave me this walkthrough that says:

The Spider Bandit can be fought after blooming the guardian sapling. The Devil Gate will be active after clearing Wawku Shrine.

Which means I can fight the spider, I have already bloomed the sapling... but I just can't dig my way down to the gate!

Any idea how I can dig this?

In order to dig there you need Cherry Bomb 2, which is, as you guessed, an upgrade of Cherry Bomb.

Go to North Ryoshima Coast ocean, on the northern, tree-shaped island. You need to donate 120,000 yen to the Divine Springs (found under a surface that must be Cherry Bomb'd) in order to receive the Cherry Bomb 2 brush technique.

After you did buy it, don't forget to take Travel Guide: Cherry Bomb 2 from the chest.

The Cherry Bomb 2 brush technique is the same as Cherry Bomb.

Once bought, go back to that spot, and use Cherry Bomb 2. Once inside bloom the typical clovers (3) and trees (4) below, then investigate the Divine Spring. Cherry Bomb 3 is the prize if you donate 300,000 yen to the spring. I don't think this ability is amazingly useful, but you will need it for a stray bead down the road.

Steps for getting into Kamui's Devil Gate trials:

• Digging Champ
• Killing the Bandit Spider
• Fighting Lechku & Nechku who are found at the Wawku Shrine.
• The third and final Devil Gate in the Devil Gate Trials is found behind a bomb-able part of a frozen waterfall at the far north end of the region, almost below the entrance to Wep'keer.
  • For a little bit of help on finding this watch 0:40 to 0:50 of this; and the first 20 seconds of this.