Description of Achievement: Score an "A" rank during any mission in Classic mode
Earn this in the Crimson Dragon Walkthrough

How to Earn:

Make sure Difficulty is Classic.

Once you're sure it is, boot up the mission "A Recruit's Duty" under Coral Lake East. My game seemed to be on Casual by default.

A side note is that once you're at the Missions main menu, with Missions, Multiplayer, Dragon Roost etc you cannot change options, you'll need to quit out the game completely (including quitting it from the dash, it can't be left in the background by simply hitting the Guide button), reboot it, and select Options from the first screen after pressing A at the main ident screen. I don't understand why this hasn't been patched, but there you go.

Click on Select Wing-man, use Largo, you're good to go. Do as well as you can, you only need to get A ranking. If you manage to get S you'll also get the achievement. My first try I got B, and obtained the "Not Bad, Rookie" achievement