While all the solutions here work.. If your going to be using split-screen co-op.. One very important issue with this one and
These two achievements only count on the primary profile (Controller 1). We spent a good 40 minutes boosting with out this achievement popping for the secondary profile. We then switched the profiles over, and tried again, this time counting the kills. There were 0 kills previously recorded, and we had to boost for all 30..