Assassin's Creed Chronicles: India

Helix Blade is an ability you can only use in Plus mode. You must sneak up behind an enemy and assassinate them using Square, but immediately press Triangle during the assassination animation. If you have performed it correctly, the enemy will disappear, leaving no body behind.

This is easily farmed in Sequence 3, in the same area as Two For One. Throw a smoke bomb at the guards, perform two Helix Blade kills, and reload the checkpoint until you get it. You won't have to wait for all the smoke to disappate to confirm that you did this successfully; it's quite easy to see the guards' bodies through the smoke.