Goal: Lose 1,000,000 points in 1 round of Black Label or Black Label Novice.
Basically, in BL mode you have a blue bar to the bottom right of the screen that signifies your ability to slow down bullets. This of course is helpful in trying to 1CC BL mode if interested at the trade off of losing points.
Whenever you have any blue gauge you can touch bullets coming straight onto you - you should notice a red square pop up over this area with a negative number overlaid. If you notice to the right of the screen under "Stage Total" there will be a negative number mixed into the equation. You want this to become 1,000,000. Also, although I am not 100% positive "round" seems to mean "level" for this game so attempt this during 1 level.
I got this by practicing throughout Black Label Novice. Whenever there are large clumps of bullets being shot at you attempt to hover in front of them and "gain" negative points. When you run out of blue gauge you can refill it by gaining gold. Level 6 is an easy way to get this due to the amount of bullets and the 1st final boss has some great clumps to exploit. I would recommend having 3 lives at the beginning of level 6 for I am unsure if continuing mid attempt will count towards this - however continuing until level 6 is fine.
All this said, it unlocked for me at ~900,000 negative points so perhaps something is off here but if you follow this idea you should easily get this achievement.