This achievement is referring to the blue "soul gauge" on the side of your screen. Each time you fill the gauge, you get a score multiplier. To unlock this achievement, you'll need 2,000 total fills accumulated over all your plays on all the characters.
You can increase the Tamashii gauge faster by destroying enemies while you are in the upper area of the screen - starting from halfway, the closer you are to the very top when you destroy an enemy, the more "soul" gauge you'll fill.
Also note that enabling Auto Bomb will slow the rate of gauge increase by a small amount.