For this Achievement, you have to play Arrange mode at level 5*5 rank or higher. The two numbers represent the difficulty rank of the character's default state, and also the rank when in Kakusei mode.
The rank level can be raised by clearing Arrange mode without continuing. Each time you manage this, you can inrease the rank by 1 in the options menu, so that'll be five times you'll have to 1CC the game before you can set the first number at the required rank.
The job will only be half-finished though since the Kakusei rank (the 2nd number of the 5*5) is raised independently by 1CC-ing Arrange mode as Player 2. To start a game on the 2P side, press the back button on the title screen.
Alternatively, you can get this achievement the easy way by just doing what's required for the "Spectator" achievement..
Only make sure the replay you download is one that's at least Lv 5*5 rank (there are plenty to choose from).
Once you've updated your profile with the replay data, you'll be able to adjust rank to the required level in the options menu and the achievement should pop when you start playing.