In order to get this achievement you'll need to find a largo where half of it is a boom slime. Boom slimes can only be found in Moss Blanket, Indigo Quarry, and Glass Desert. Boom largos will occur naturally in these areas, but if you want you can carry around a plort to launch at them one once you've found them. From there, simply grab the largo with your vac pack and launch it into the air! It may take a couple tries before it actually explodes in midair, and that's it!
Here's my video for all achievements if you'd prefer. Skip to 1:45 for Fireworks.