Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. To change the names of levels, open the "title" file located in the "/english/pc/game" folder. Then, change the name of a desired title.
While on any level and there are two enemies talking to each other, toss a grenade (for example at the beginning of the Think Like A Gun level). They will not even look to see what it was because they will be distracted.
Certain enemies hide in areas where you cannot shoot them. Toss a grenade at them and they will jump out of hiding. When they jump out, they will land on the ground on their stomachs. They will require a couple seconds to get back up. Before they do, aim and shoot them or they will go back and hide again.