It's possible to get most of the medals on the very first mission. You've got to stand a little bit out of the way of the Grunt groups that come in, and just chuck grenades. There's at least five or six grenades on the map, so it's possible to get it by killing at least two Grunts with every throw. If you mess it up, you can always restart the level since it's pretty short.

Hope that helps :)