This is most easily done in the later missions where there are more enemies. Play on Easy. Then go into a level, and use each characters frag grenades. If you took out atleast one enemy with each of these nades, your grenade count should be up to six. Then use the incendiary grenades using Lang. If you kill one enemy with each of these, you are up to nine. Then just keep an eye out for a supply point to replensih your grenades, and once you have a new set, this should come quick.