Assassin's Creed

Widely accepted and believed to be unlocked by:

"The DNA bar is the bar that fills up as you complete the investigation missions in each memory block. In order to get this achievement, simply complete all six investigations for each assassination rather then stop after three."

From Microsoft:

"Again, this achievement really only requires awareness and patience. You need to assassinate every target with a full DNA (health) bar, not just kill them in open combat. Make sure every assassination is a one-hit kill with your hidden blade. This doesn't mean that every assassination must be a low-profile hidden-blade attack though. Holding down for a hidden-blade strike for a high-profile assassination will still score this achievement."

-Link-

This achievement is currently glitchy. It has been confirmed that the above will get you the achievement, but less also will. Also, if you have every side mission done before you start the assassinations, it will unlock. For updates, check this -thread-