Assassin's Creed Chronicles: China

You will unlock the ability to use the Helix Kill once you have finished the game through once i.e. at the end of Memory Sequence 12: Vengeance. A new meter will appear below your health bar at the top left of the screen. This is your Helix meter and it can be filled by picking up Helix shards (see screenshot below). In order to perform a Helix Kill black at attack by pressing + , and then press + at the same time to engage Helix Kill mode. When an enemy is in range press or to deal a single killing blow. You should focus on these kills during your second playthrough of the game or once you have obtained the high score for each Memory Sequence as combat kills do not reward you with many points towards your overall score. Once you have killed 100 enemies using the Helix Kill, the trophy will unlock.

A good place to grind these kills is in Memory Sequence 6: The Search. Once you pass through the gate, get alerted by the enemies on your right and 3 more enemies will spawn to your left, totalling 5-6 enemies. Activate the Helix Kill and and kill all enemies. Restart the checkpoint and complete the same thing again until you have unlocked the trophy. See Smash And Stab for a video.

Screenshot showing what the Helix Shards looks like in the game.