Killer Trophy in The Psychoduck
Complete your first level.
How to unlock KillerStory related. Cannot be missed.
The key to every level in The Psychoduck is to sneak behind the heron by crouching with and sneaking up behind him so he won't notice you and press to stab him from behind whenever you are near him to advance to the next stage. Otherwise, he will flee the scene once he finds out you are approaching him provided that you didn't duck and tried to approach him by normal means.
Needless to say, this trophy will be unlocked upon completing the first level of the game. You can also unlock Malacologist upon collecting the snail that's present in the level.