An easy spot to grind this is in Mission 2: Operation Cobra. After the long tank drive at the start of the mission, you will have to go in an enemy building. In the very first enemy building of the mission, head upstairs and just stand in the doorway, blocking your allies from moving forward. Enemies should throw grenades at you, simply throw them back. This is cumulative, so you can die or reload a checkpoint if needed.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKe25WzwEqI...