How to move and rename all files from subdirectories to current directory?

Question

I have a folder that looks somewhat like this:

Course
  Lessons 1
    a.mp4
    b.mp4
  Lesson 2
    c.mp4
    d.mp4

I would like the end result to be this:

Course
  1. a.mp4
  2. b.mp4
  3. c.mp4
  4. d.mp4

I managed to achieve some minor success using LADB's app and executing a bash script, but this is not practical since I always have to re-run the app.

Are there any simpler solutions?