This method worked for me.
I just played through the game as I looked for missed journals and used grenades when they bunched up, sometimes on lone enemies too. I didn't keep working the same enemies or reload any checkpoints. It seemed like it took more than 10 but not an absurd amount more. I never really kept count; just took the opportunities when they came and cooked the grenades before letting them go above their heads.
-Insert something creative here-
( that doesn't include stats that no one really cares about or some poorly photoshop'd image with your name stuck in it. )