This can easily be achieved on the second chapter of Act 1. There is a large amount of Wretches that spawn once you reach the down Raven. Wretches spawn usually in groups of 4 and will stay together. You will just have make sure you have the right lines of sight that if you hit the first you will hit the second. I would suggest a heavy unit, as their mulcher will easily go through two wretches as seen in the video.