Stealth takedown 15 enemies in a row.
There are many places you can do this in chapter 4, which is what we recommend, but the Trident Fort at the top of the waterfall may be the best place. It has plenty of enemies as well as reinforcements that appear on your way out. It makes up more than 15 enemies.
Attempt this achievement on explorer difficulty – it takes longer for enemies to detect you – and stick to the long grass for cover. If you happen to get detected, just remain hidden until they stop looking for you.
You can also track your progress from the statistics menu under Hand-to-Hand combat.