DEATH BY CUBE

Even easier than Lunars solution is to use dash on the same level (World 1-Level 3). When the level starts avoid everything until you feel there are 130 small cubes on the screen. Grab the laser and chocks away...

Here a nice video I found for the achievement. They show 3 solutions and its the first in the video. Really helped me a lot. Big thanks and credit do to Rooster Teeth.

Since nobody's seemed to have found a better spot yet, I'll just point out that I got this in 01-3 using the character Dash. Survive far past the gold medal (I had around 6 or 7 million at the time, though it will likely vary depending on your multiplier.) and eventually an absurd number of the small cubes will come after you for a while and keep respawning as you kill them, during this time a laser should appear, grab it, and you'll get this fairly quickly.