The idea of this achievement is simple:
You must do every possible quest (saving citizens, pickpoketing, evesdropping and etc.) plus you must synchronize on every High Point.
You can check the status for them in pause menu-memory log. It will be looking like a DNA section.
Every mission is complete when only 2 bars are black out in DNA (one for completing the preparation the other for successful assassination).
Only then you can assassinate your target.
if you will act this way through all memory blocks, in the end, you will get the achievement. Because your DNA bar will be complete for sure :)