I have an trick to kill all the mimics without making an single mistake:
First, you must scare everyone with an bullet(without killing them) so EVERYONE(don't let ANYONE to go anywhere else than the corner) go to any corner. Continue scaring them to the corner until only three persons remain. The three persons remaining are the mimics.
Edit: Please pin this comment so everyone knows how to find and kill the mimics. Thx :)