This will be a very boring 10-12 minutes of your life, be warned.
You will need to collect 300 mushrooms to get all of the achievements, which is quite easy to do, just very very boring to execute.

Mushrooms Collect 1
Mushrooms Collected 1


Mushrooms Collected 2
Mushrooms Collected 2


Mushrooms Collected 3
Mushrooms Collected 3


...
Mushrooms Collected 300
Mushrooms Collected 300


There are 300 achievements, each for collecting another mushroom. I'll walk you through each step of easily getting this spam hundo.

Step 1. Press "Play" on the main menu















Step 2. Select level 1 on the level select menu















Step 3. Press the "K" key















Step 4. Go up to the purple mushroom (to the right) and press "Enter" when prompted. Keep going to the right after this.






























Step 5. Once you get to and obtain the yellow (fourth) mushroom which is next to the translucent green wall, press the restart button at the top right, in the middle of those three.






























Step 6. Repeat the aforementioned steps 74 more times.
You'll need to do this process a total of 75 times, each time granting 4 more mushrooms. Keep doing so until the counter in the middle at the top of the screen reaches 300/21 mushrooms. Then, you're all done!

Hopefully, you've made it through the 10-12 minutes of pain that is collecting 300 mushrooms, and gotten the hundo! Congratulations, if you can even consider this too much of a feat...