This is for getting 30 kills with the grenade launcher. Load up chapter 10 and kill the guy with the grenade launcher at the top of the stairs in the red shirt. Proceed forward without using it and get past the quick time event. There will be 3 guys inside the building to your right. Use the grande launcher against them and restart checkpoint as necessary.
Also you should pause and save game after getting this so that the game keeps the medal points.