"Exalt" come with the Enemy Within expansion to the game. For this achievement, it should be noted that mind control won't work for this. You can force an enemy to commit suicide by using a standard Arc Thrower to capture an EXALT soldier. When you try, he will auto-suicide. The Arc Thrower does not need to be upgraded for this. Here is a video to aid you if you are having issues with this achievement:
Credit goes to MrReign for the video