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. . .
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!
Play with Death Epileptic Pixel mode on. (There's also a game of the same name)
Get below 8:00 in speed run mode.
Buy all the costumes!
Beat the dev's score on the last level! (You might want to shoot for around 1500)
Unlock all the levels!
. . . or it would have been, anyway!
Well, it's still there. . . just less dire.
I told you to go right, didn't I. . .?
Well, it's sorta' in the level. . .
It's there, it's unique! Just look closely!
Everyone knows it's the best part!
It's a practical paradise, just for you!
Place fifty-jillion (100) plants in the bounce house at once.
Looks like someone just likes firing the Rainbow Gun. . .
Convince/bother/bribe the dev until everyone gets this achievement.
Be addicted to social media.
Have the game run below 30 Frames Per Second.
This one's not too hard to find. . .
You're the best! :D