Here is the full list of all 21 Rubber Ducky and the Rainbow Gun achievements.
Get below 8:00 in speed run mode.
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)
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.
Play with Death Epileptic Pixel mode on. (There's also a game of the same name)
Unlock all the levels!
It's there, it's unique! Just look closely!
. . . or it would have been, anyway!
Place fifty-jillion (100) plants in the bounce house at once.
Buy all the costumes!
Be addicted to social media.
I told you to go right, didn't I. . .?
Well, it's still there. . . just less dire.
Well, it's sorta' in the level. . .
Looks like someone just likes firing the Rainbow Gun. . .
It's a practical paradise, just for you!
Everyone knows it's the best part!
This one's not too hard to find. . .
Have the game run below 30 Frames Per Second.
You're the best! :D