Simple and Easy. You will get this as you naturally progress throughout the game. I got it about 3/4th the way through the campaign.
Try to do every mission possible to speed this up. I found that crashed UFO's were where most of the enemies are and the best for getting a ton of kills.
Also to make this easier, make sure you do a TON of research and invest in better weapons and HOVER SHIVs. SHIVs with the plasma cannon take most baddies out easily if not cripple them.