This achievement can seem a bit daunting at first especially if you aren't the best at twin stick shooters, but fear not!
The level "Wrecking Ball" (Level 11) in challenge mode is a fairly easy one.
The wrecking ball is good because it can help protect you when spinning it around and also if you get kills with it, it drops multipliers for you to pick up.
This game has a lot of different types of a modes and levels so you may find a certain level suits you better, but personally I found this level best for it.
Here's a video of the level you can get a 15x mulitplier on: