Easy enough to replay some of the ODST missions to collect the requisite number of Engineer kills, but important to note that you don't actually need to be playing H3: ODST to complete this achievement. After playing through ODST, if you still need some Engineer kills for this achievement, you can rack up some more in Halo Reach in the New Alexandria mission. At least two Engineers are in the hospital, and a number of extra Engineers may be available depending on the randomized side missions during New Alexandria.