Story related and cannot be missed.
There are 8 memories in DNA Sequence 6, 6 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - Into the Shadows
- Assassinate a guard from a hiding spot
Memory 2 - Honor, Lost and Won
- Perform an air assassination on Tarik
Memory 3 - Bearer of Mixed Tidings
- Do not kill anyone
Memory 4 - A Little Errand
- No optional objective
Memory 5 - Signs and Symbols, Part III
- No optional objective
Memory 6 - The Maiden's Tower
- Obtain the Masyaf Key without falling into the water
Memory 7 - The Mentor's Return
- Do not kill anyone except Abbas and his Captains
Memory 8 - Setting Sail
- Do not kill anyone with blades
Once you complete Memory 8, the achievement should unlock.