For disassembly, there are probably many ways to undo that castle nut.
For assembly, the proper tool is a so-called "pivot adjust wrench", Honda P/N 07908-4690002.
This wrench allows you to hold the adjuster bolt with a hex head socket through the wrench while tightening the castle nut with the wrench. The specified torque of 8.0 kgm is only correct when used with the special wrench (it has an offset of ~ 4 cm). If you use a normal wrench one would have to calculate the correct torque.

How do people do this without the wrench?