I like #12 and #13 the most. Especially the envelope with crown. Mastery means "comprehensive knowledge or skill". It would be great if the logo expresses that. However, I think the logo can be improved by playing a bit with the letters and the font.
The other entries still look a bit sketchy to me. In #19 I do not see the letter "e". In #18 I would use red for the "flag". And the abstract icon in #16 looks a bit too complex for me.