The "Frag Master" trophy can be unlocked by performing any of the following three tasks:
During the battle of Khe Sanh when the NVA are pouring out of the tunnels in the background, throw grenades into the crowd. You are bound to kill some of them.
Get five of your friends to join the other side. Tell them to cram together, then toss a frag grenade.
This requires two players. Blow the legs off five zombies and have the other player to stand in one place while the zombies crawl towards them. When they are all attacking your friend, throw a frag grenade.