Guida contratto del Mutaforma in The Witcher 3 Wild Hunt [PS4 - Xbox One - PC]
The solution for this particular side mission of the game.
Trophy Guide / "Infiltrator" Objective
To unlock this trophy, you must complete the mission "An elusive Thief" (an elusive thief) which can be started from the mission bulletin board in Hierarch Square in Novigrad. From the bulletin board you will have to take the note "Contract: Imp" (little devil).
Go to the market square by day and talk to the merchant, then follow the red footprints on the ground using the witcher sense.
You will reach a house where you will have to take a letter, read it. Follow the trail of clues again to finally get to the shapeshifter. After defeating him you will have to choose whether to kill him or let him go. If you choose the second (recommended) you will get a lot of coins and a diagram, then go back to the market place again and talk to the merchant to conclude and unlock the result.
Here is a video showing how to complete this contract, good luck!