This achievement is actually pretty simple and can be done on any Level on any difficulty and at any speed.
If you look at the top left of your screen you will notice your number of lives "ships". Under this is a small ship icon and you will notice as you contract and expand your ship less/more symbols will appear.
What you want to do is expand your ships until you see 9 of these symbols, this is considered a max spread. Now simply kill any boss while in this spread for the achievement.
Bosses can get pretty cluttered, so if its easier for you, you can whittle the boss down to low health, then spread out right at the end to land the killing shots.
Achievement pops after the level ends.