You need to score 10 million (10,000,000) points off of one enemy.
If need be, easily done in training.
At main menu, choose "play game" -> "bullet soul" -> "training"
Stage 2 (or 3-5), Ship Stock infinite, shot level 4, bomb stock 3 (or 0).
Play as Yun.
Since you have infinite lives you'll never lose your multiplier, and yes this particular achievement unlocks in training.
For everything except the mid boss and boss, stay as near the top of screen as possible and kill enemies as quickly as possible as they enter the screen to increase your multiplier.
Once you get to the boss, the general strategy is to lock on with the lasers by holding X (by default) but then move to either side and then up and down the side as need be. This will avoid damaging the boss with the forward laser and gain many points. Near the end of the boss fight, this achievement should unlock if done correctly.