This achievement can be earned in Siwa, the first territory of the game. While there are other places across the game this can be done, the Coral Escarpment Camp in Siwa had exactly 10 low-level enemies. This one takes a little patience. Take your time scouting with Senu and then move methodically from enemy to enemy so you aren't detected. If you haven't gotten the hidden blade yet, your takedown move won't actually kill enemies so you'll have to follow up with a quick attack once they're down to finish them off. Once you kill your 10th enemy without being detected, the achievement is yours!