This achievment most likely needs some farming. I had almost everything done and played through all of the episodes multiple times and still only had 20% of the achievement.
I think the fastest way to get it is in Barry's chapter of episode 2. In that episode, you don't need to play till the end of it. If you rush through the first part of the level (roughly 10min in the episode) you'll come to the spot at which you separated from the male character in Claire's episode. Go through the blue door inside the building and you'll find yourself at a staircase. Go all the way down and through the door. You' ll find an enemy with a weapon arm right in front of you. The neat thing about this method is, that your checkpoint will be right in front of this room. That will make it incredibly fast and easy for you to get the 30 kills. I recommend using the Magnum for this. Don't worry about ammo, since you are going to abuse the checkpoints. Aim at its arm, slightly above the weapon part. You should destroy it with one or two shots. Rinse and repeat!
(Video credit: Nathan Summers)
Hope this helps!
Any improvements? Please let me know in the comments below!