Android - How to delete photos ONLY from google photos backup?

You can uninstall the "Google Photos Backup" from your device or simply sign out from the account in the Photos app. Then go to the web-version of Google Photos (photos.google.com) and delete the photos.

• Go to photos.google.com and login using the Google account you're using on your device.

• Place your cursor over items you want to move to trash. At the top
  left corner of each item, click the select icon . (Or hold down SHIFT to select a range.)

• At the top right, click the trash icon.

I tried it personally (after signing out in the Google Photos app) and I'm sure it won't delete the photos from your phone.