The One Percent
The Elites are usually the leader of any group of grunts and jackals, in legendary they aren't very easy to kill, killing 100 on any difficulty will unlock this achievement.
Every elite counts, here is a picture of basically all elites and will count towards this achievement.
Since Halo 4 is mostly about Prometheans you will kill more elites in Halo 1-3.
And I also found a video done by Machinima somewhat educational and funny. (If you don't get the joke in the beginning its because elites guard a lot through-out the campaigns).