I had no hanging grenade kills from my first play through.
Still mopped this up within twenty minutes.
Load up Chapter 4 - Plane Wrecked.
Go through the gate, up the waterfall and take out the bad guys shooting you from the ledge. Cross the log and climb the vine. Take out any more nasties at the top, and behind the wall in front of you is a crate with two grenades - be sure to pick them up.
Go down the narrow corridor taking out the three or four bad guys on the way and you'll come to a large area with some ruins, and several dudes wandering around. Clamber down, and just before the arches you'll see a short stubby wall standing on it's own.
You can hang off this wall and lob grenades over it, there'll almost always be a few bad dudes within range. Chuck your two grenades for two kills, then reload the checkpoint. You'll go back to just before the large area again, equipped with your two grenades.
Keep going, and within five or six reloads you'll have this in the bag.