Batman: Arkham Origins
49 Achievements
80-100h
What hit me?
Take down 100 enemies who didn't know you were there
7.4%
How to unlock the What hit me? achievement in Batman: Arkham Origins - Definitive Guide
(ironfelixxx points out in the comments that he was spotted but still got the achievement, so I may be wrong about it not counting if you are spotted but escape)
The easiest way is to get silent takedowns. You will get a lot of these during natural progress. If you need more, I recommend flying around the city searching for rooftops that have two men on them with guns. There are a lot of these rooftops throughout the map, take one guy down silently while the other isn't looking then finish off the other guy for two easy kills.
The most efficient way to get this is to just attack rooftops often while searching for collectibles to avoid grinding, I got this achievement simultaneously while collecting Riddler data.
The way I earned this trophy is through silent takedowns. To perform a silent takedown, make sure you are standing or crouching behind a thug or police officer and press on your control pad. Batman will perform a choke hold from behind and the enemies will slowly, but surely pass out.
You can earn this trophy on a regular playthrough, but if you haven't gotten it afterwards, just fly around Gotham and look for thugs patroling on rooftops. Make sure they are isolated before you do a silent takedown.
Make at least one-hundred thugs fall wihout them knowing you were there and the trophy should unlock.
Happy hunting and good luck!
This is cumulative, as long as you stay quiet through all the predators you should get this by the end of your third playthrough. See Silent Knight for tips on taking down as many enemies as possible silently. If you don't have this by the end, go around Gotham and take out enemies on rooftops without them spotting you until you get the trophy.
This achievement is relatively straightforward - you will simply have to take out, by any means of silent takedowns or gadgets, a total of 100 enemies without ever being detected. These silent take downs will most likely come naturally as you encounter the various "Predator Rooms" within the story.
If you have not yet unlocked this by the end of the game, it's highly recommended to glide around Gotham and search for rooftops that have 2-3 armed thugs on patrol. They are usually spread out, which means you can get an easy silent takedown on each. Most of the rooftops in the city will be populated with thugs, so just keep an eye open and make a stop on each rooftop. There is no set way to keep track of this achievement, so just keep performing silent takedowns until the achievement finally unlocks.