The Super Elites are the very large, slow, machine-gun wielding enemies that are just a pain in the *** to kill. They can be found in both game modes, but if you are looking to boost this, horde will be your best bet. These suckers take 8+ headshots to kill even on the lowest difficulty setting, so if you are playing with randoms I would highly recommend a fast firing rifle like the Gewher, M1 Carbine, and M1 Garand.
In the No Sanctuary level, they spawn quite frequently.
If you are having trouble actually getting these kills or you are just impatient here is a glitch that you can take advantage of....
If you begin a match with two or more people and they leave, any Super Elites you kill still count as being done in co op. How you take advantage of this is up to you, but odds are at some point you will be playing Horde and someone will quit. If this happens you will be able to continue on and have the match continue to count towards this achievement.