1 0 0Kills do not carry over between levels of a chapter, so you have to get the 50 kills during one level. I got it on the last level of chapter 2, which is the first level with enough enemies to get the trophy, while playing on normal difficulty.
0 0 0The kills may not carry over from chapter to chapter but your kill streak will not reset if you reload checkpoint. At the start of chapter 2 you will be in a office with green and white walls just kill the 3 sets of enemy's at the start and then reload checkpoint after a few restarts 1v50 should pop