Walk up to enemies and when prompted for a "Kill Move" hit "A" and you will execute them in an action sequence. In the first part of most missions the enemies are unaware of your presence so run up to them and hit "A" to get some easy kills.
You have to play through the campaign 3 times so you should get 30 pretty easily. On the lower difficulty settings you can pretty much just run up to every enemy and execute a kill move on them without getting killed.