Rockman ZX Undub patch v1.01 is made by Geo a.k.a. illuminor.
GBATemp thread: https://gbatemp.net/threads/492408/
Feel free to contact me on Discord: Geo#4593.

The included patch for enabling Rockman Zero 3 & 4 extra bosses
without the need to insert Zero 3/4 cartridges into Slot-2 is made by Prof9:
https://github.com/Prof9/MMZX-Slot2-Patch


To apply the patch on Windows:

- Place a clean Japanese ROM file of Rockman ZX (CRC32: B577BBE0)
  into the patch folder (where this !README!.txt file is);
  
- Rename your clean ROM "zx.nds";

- Run "Patch ZX.bat";

- Done! You will now see a file named "zx_undub_v1.01_zero_bosses.nds"
  next to your original file. This is the undubbed ROM
  with Zero 3/4 extra bosses enabled, you can now play it.

Good luck!

If you're on an operating system different from Windows, I'm afraind I can't provide an instruction for you.
You'll have to google how to apply VCDIFF patches using xdelta3 on your own. It's definitely possible.