Must be obtained in a single playthrough. Returning to the title screen, through quitting or death, resets the progress to zero.
Setup:
Pick the first guy as your ship. Hold to fire your primary weapon, release it to move faster, and tap to perform a diagonally forward dodge in the direction you're strafing. This allows you to pass through objects unharmed, with good timing. Occasionally you'll find a power up which you must dodge into to collect. Sometimes dodging will leave a green arrow on the screen - press again to perform a chain dodge, often destroying things in the process. If you ever become a frog (!), your dodge becomes much shorter - prepare for this.
Method:
Everything you kill and collect adds to your score. Kill everything and progress as far as you can. The score doesn't reset if you have to continue, either! As long as you aren't returned to the title screen you'll be fine.