You need to kill two Monster on which you have a Contract on, only with your Silver Sword.
If you haven’t done this is White Arden, then the Contract Shrieker (Level 8) and An Elusive Thief (Level 13) in Velven/Novigrad are not that hard – also both Contract have their own Achievement. I did this with Level 11 and the Griffin Witcher Gear, pretty easy.
Are also good targets for this Achievement: The Merry Widow and and Muire D’yaeblen
A List of all Contracts and where you can find them is here