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. . .
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!
Play with Death Epileptic Pixel mode on. (There's also a game of the same name)
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!
Convince/bother/bribe the dev until everyone gets this achievement.
I told you to go right, didn't I. . .?
Looks like someone just likes firing the Rainbow Gun. . .
Well, it's sorta' in the level. . .
Everyone knows it's the best part!
It's there, it's unique! Just look closely!
Place fifty-jillion (100) plants in the bounce house at once.
Buy all the costumes!
It's a practical paradise, just for you!
Well, it's still there. . . just less dire.
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
Get below 8:00 in speed run mode.