HowDoIGetThisAchievement.com

It's okay, I gave up looking too, that's why we're here isn't it
lots of spoilers ahead, so proceed at your own risk, you've been warned.

Bongo Cat

Open the game and click or type 1 time
With the game running in the background, click or type 10 times
With the game running in the background, click or type 100 times
With the game running in the background, click or type 1,000 times
With the game running in the background, click or type 10,000 times
With the game running in the background, click or type 100,000 times
With the game running in the background, click or type 1,000,000 times