Here is the full list of all 21 Rubber Ducky and the Rainbow Gun achievements.
You've defeated the derpy brother and unlocked a message from the developer. . .
Get below 8:00 in speed run mode.
Beat the dev's score on the last level! (You might want to shoot for around 1500)
Play with Death Epileptic Pixel mode on. (There's also a game of the same name)
Place fifty-jillion (100) plants in the bounce house at once.
Convince/bother/bribe the dev until everyone gets this achievement.
Type the 2nd section of the dialog from the 6th level backwards into the cheat code selector! Don't forget to puncuate and capitalize properly!
Buy all the costumes!
Unlock all the levels!
. . . or it would have been, anyway!
I told you to go right, didn't I. . .?
Everyone knows it's the best part!
Well, it's sorta' in the level. . .
It's there, it's unique! Just look closely!
Have the game run below 30 Frames Per Second.
Looks like someone just likes firing the Rainbow Gun. . .
It's a practical paradise, just for you!
This one's not too hard to find. . .
Be addicted to social media.
Well, it's still there. . . just less dire.
You're the best! :D