Memory Sequence 8:
At the very end of this level you will come to a large room and see a cutscene. Now enemies are spawning in this room (that's the ambush). Your main objective is to kill all 7 soldiers in the area. Doing so will end the level. Kill all enemies without being hit and the trophy / achievement will unlock upon killing the last one.
If you get hit you can simply restart the checkpoint. Kill the enemies on the upper floor first and then work your way down. Perform some quick slide- & air assassinations to take everyone out quickly and avoid being seen.