This counts across reloads and deaths, so once you find a spot, you can farm it easily. For me, the good spot is mission 6. Head to the left at the first fork and leave the artillery alone. Once you have a good angle on the artillery, Save, then shoot the explosive barrel on its left. This won't kill either operator, but ignite the explosives under it, earning two kills towards this achievement. Repeat five times. Here's one of my runs.
https://www.trueachievements.com/gameclip.aspx?clipid=741646...