The Achievement is located at the end of Memory Sequence 8. The Mainobjective here are our 7 Enemies, so after you kill the last one, the Sequence is over, keep this in Mind!
If you get a hit from a Enemy, simple reload the latest Checkpoint and you have a new Chance. You will start directly at the Ambush. The best Way is assassinate all without being seen with Slide and Jumpkills. The Achievement unlocks in the ending Screen of the Sequence.