During Mission 6 - where you're playing as the lady assassin, towards the end of the mission you'll have a 'Helix Code Meltdown' moment. When you're back in the the game you're taught how to Helix Blade dissolve enemies.
Press
followed swiftly by
when you've started the stabbing. Earliest point you can do it. Good thing is, as soon as you've killed the first guy, reload checkpoint and repeat. Simple. You'll also pop this achievement a few minutes later once you've cleared the house of enemies:
Just in case you don't read comments, here's a 'mop up' method after you've finished the game from
KugareSome people (like me) keep this achievement for the "mop up" part at the end, because there is a chance to unlock this achievement without grinding and using a guide. I was only a few helix blade kills away and i was looking for the fastest way to unlock it. "towards the end" means i had to replay a big part of the mission - so i was not going to use your solution.
I started mission 6 (plus mode) and killed the first guard i met (he looks out of the windows) - i let him disappear and reloaded the checkpoint until i got the achievement.