DEATH BY CUBE

I tried a few other solutions that would always lead me to the same result: Getting no way near the 400 mark.

So if you're like me try this method, which I used to get to 999.99 on my second try ;p

Buy the Dash character and choose level 1-3. Now wait for the first wave of enemies to appear and use the dash to get out of the circle. Hold LT and you will be invisible for a few seconds. All you have to do now is wander around in a circle and use Dash every few seconds. The enemies will always go to your last known position, before they spot you 3-4 seconds later.

When the screen is filled up with enemies, dash in the direction of the laser and blast everything away. This first attack alone should net you well above 200 (And also nets you another easy achievement). Repeat the first step and wait for the next laser to appear. Use it and you should be at around 300-350. Repeat the same strategy again and you should have a 400x multiplier.

Here is another video for this game. They show off 5 achievements and this one is in the middle. Helps a bit to see how it can be done.
Thanks and credit goes to Rooster Teeth.

This can easily be attained fairly early on by purchasing the 'High Risk' character and playing the stage 01-3. It will take a bit of practice to be able to avoid the speedy little cubes, but after a few tries you should be able to kill 399 of them. (High risk will 1-hit kill them and give you a +1 to bonus rate no matter how far away you are from them, but you will die in one hit).

Save the first laser for when the second wave of cubes shows up, as it's a waste on the initial batch since there will be a long delay until the second. From there just run around the map making use of your dash to go through enemies that are about to touch you and cluster all the cubes on your shadow image for easier kills.

It is easiest done in IO's world. Level 01 - 3. Use Dash [The yellow and black one.] Who is 1600 credits. Use him and get to the bottom left corner and use the Dash across all the way to the right. Shoot the stunned targets. Rinse Wash and Repeat. Achievement Unlocked. Happy Hunting.

Thanks to Kronic1018 for heading down an easier path. The Spread character is a lot easier to use for getting this achievement. Go for multi kills so you can upgrade and max out your spread weapon when you collect those pink spread upgrades. It covers ~3/4 of the area around you when maxed out so you can pretty much sit in a corner and just fire without worrying about dying.

It takes much longer than it would using High Risk, but it's a lot more managable with spread.