Astropop

The best way to get this achievement is to go in to survival mode with any character. Almost immediately at the start you will get a block that will destroy all the blocks of a particular color (a 'nuke' block). Organize the blocks so that you have 3 together and are ready to shoot a fourth as soon as the screen fills up to its maximum, making sure one of the four is the nuke block (credit to Jellophile for suggesting that method) and you should get this within a few tries. I got a 15x combo twice, then got an 18x combo my third attempt, so it shouldn't take too long. This is much faster than having to go in to level 9, beat it, and hope to get it during level 10 like the other solution says. This only takes a minute or two per attempt. Plus, if you are like me and didn't see that solution until after you passed the level 9 checkpoint with Vector, you would have to start from the level 1 checkpoint, taking even longer.

Note that you can immediately restart if you don't get it, or you can keep going and another nuke block will usually appear shortly afterward, allowing you another attempt without having to exit out.

Get to level 9 with Vector, the game will checkpoint.

Start from level 9 checkpoint, finish 9 and get to level 10. The game will give you a nuke brick at the top of the screen as part of the tutorial, just wait until the screen is filled with bricks instead of using it straight away.

Try and keep the bottom layer of bricks flat so theres more of them when you reach the bottom, but make sure 3 colours are connected so you can just fire the nuke at them.

If you fail, exit and start from checkpoint level 9 again. It will take some luck, but this is the easiest level to do it on imo.

The way I got this was by getting the special block of a color (the one that will destroy all of that color when used) and a set up where I can easily use it. Then I waited until the screen was almost completely full, and launched it. Hoped for the best. Took a few tries, but I eventually got lucky.