Now I might just be a terrible singer, but this achievement was way harder than it had any right to be for me - I finally got it on my 150th song to complete the game, after 15-20 118k songs
To get this, you need pretty much back-to-back perfect scores. Expert mode (no lyrics on screen) is also required, as the score appears to add about 30% when this is enabled.
I'm unsure whether all songs have enough for this - I finally achieved this on Sia - Chandelier, and did so by using two mic's (one hit just below 119.5k, the other 120,123) to ensure what one mic missed, the other picked up.
A big thank you goes to
https://www.trueachievements.com/Mr+Mascal+77.htm for confirming this was indeed possible to me - please let me know any questions & I'll try my best to help!