So I am 100% positive I know the trick. I went to a song I haven't played in a LONG time that wasn't mastered, was tough since I have probably 85% of all arrangements mastered in the game. It was tricky to figure out but I did it, and it worked on every time. Now the question is what to do with this information I have. For me it serves a worthless purpose I want to level up arrangements naturally and earn it. I admit I got it on Star Spangled Banner, but I didn't exactly know how, but now I do. I feel like if I give out this information it could ruin the weekly challenges which are fun for everyone. I don't see the benefit of this information coming out. What do you guys think? and trust me when I say this it is worst than I originally thought it was.
Edit: Go into riff repeater and click on leveler, then click on set mastery and lower it down to the lowest % you want. Level it up once then set mastery to a higher percentage (preferable something you are capable of playing the song at for your own ability.) It can be any percentage and then fail at it assuming you are not capable of playing it at that percentage to level it up any further. Then quit go to game setting click on highest achieved settings and then it should be set at whatever percentage you failed at, whether it was 100% or lower.