Achievement can also be unlocked with the difficulty on "practice".
i set my game up as~
Stage type: Fixed
Game mode: Normal
Difficulty: Practice
Medal: On
Credits: 1
From there just play as you normally would but just make sure you don't crash into anything and you'll be good to go. Also remember to make sure your on the first Raiden Fighters. The achievement should pop up with your final score and initials.