Easy Panther Style
Take a look at the required level you need to hit for normal: IE 3000 points for a level.
Now for the trick. Start up that level playing it on Prefectionist. All the silent kills are worth more. Once you hit the required amount of points for Normal (check by hitting start and seeing your points so far for the level), get to a check point and change the difficulty down to Normal.
You will be asked if you are sure you want to change the difficulty, choose yes and you will restart the level at the check point you reached. Your points will carry over from Perfectionist now to Normal.
Finish the level and the Panther style for that one is complete. You can also use this trick on Co-Op missions (that have check points) or with Assault and Ghost styles.
Co-Op levels played also count towards the seven, so get a partner if need be and aim for chain kills. You can not do the change difficulty trick, however, you and your partner's scores combine at the end of the level.
Easy to do Grim's Missions and Kobin's Missions on Panther and Assault.