As of writing, these "master ____ as a secondary job" achievements are potentially glitched (in a good way) because I've been unlocking them whenever my characters master their primary job. Mastery means that you have learned all available skills for a character. The final one always costs 5000 JP. And again, at least as of right now, you don't need to have it as a secondary job for it to count for the achievement. I unlocked this when I bought my last skill on Olberic the warrior.