Assassin's Creed Chronicles: Russia

You have two options for this trophy. You can either use this as Anastasia starting in Sequence 6 of your initial playthrough of the game, or you can use it during your Plus playthrough as Nikolai (or Anastasia) any time you have energy in your Helix (blue) bar. To perform a Helix Blade kill, sneak up behind an enemy, press Square to assassinate, then quickly press Triangle during the animation. If you have performed it successfully, the body will disintegrate. 

If you want to get it out of the way early, you can easily farm this trophy right after you first obtain the ability in Sequence 6 with Anastasia. You will pick up a piece of Helix, hit a checkpoint, and then be told to specifically kill the nearby guard with the Helix kill. Kill the guard, then reload the checkpoint until you get the trophy.