There are 6 memories in sequence 13 and they are all relatively short. All you have to do is complete all 6 memories for this achievement but if you are going for the "Hydrogen Bonded" achievement as well you will have to complete all the optional objectives while completing these missions. Mission list and optional objective list are as follows...
Memory 1: Buried Words
don't get into conflict
get 3 kills while in cover
Memory 2: The Book Thief
don't get into conflict
get 5 perfect parries
Memory 3: A Shadow From The Past
pass over/under 3 objects
Memory 4: Raising the Dead
don't get into conflict
get 1 air assassination
Memory 5: Under Lock And Key
3 rooftop enemies killed
2 headshots
Memory 6: A Crown of Thorns
get 2 double assassinations
parry 3 attacks
The Final "boss" you have to kill is no stronger than a normal leader. A simple smoke bomb will disable him giving you the opportunity to wail on him with the sword.