Story related and cannot be missed.
There are 3 memories in DNA Sequence 8, 6 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - Discovery
- If attacked, kill 5 Guards in close combat without getting hit
Memory 2 - The Exchange
- Rescue Sofia before she loses 50% health
Memory 3 - End of the Road
- Kill 20 guards
Once you complete Memory 3, the achievement should unlock.