To earn the trophy that's called "What hit me?" you have to take down one-hundred enemies who were clueless to your presence.
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!