After much work, I've finally figured out the method of firing slimes through the ground. It works in any flat area for largos.
Step 1: Fly up as far as possible.
Step 2: Release the spacebar (and get lots of momentum).
Step 3: Right before you hit the ground, throw the largo you are holding at it.
The largo will go through the ground.
It also works on bridges, and tarr.
For throwing a slime through the ground, you will need an area that when you slip off of it, you get insane momentum.
Step 1: Get up HIGH
Step 2: Get up HIGHER
Step 3: Get to your area of HIGH MOMENTUM
Step 4: Throw your slime through the ground
Now here's a video of me firing them through the ground, as the forums don't want to let me post gifs.
https://youtu.be/rgmhxuu51C0