In Campaign mode, during the battle of Khe Sanh when the NVA are pouring
out of the tunnels in the background, keep throwing grenades into the crowd
until you get the "Frag Master" achievement.

In Multiplayer mode, have five friends cram together, and then throw a
frag grenade to kill them all and get the "Frag Master" achievement.

In Zombies mode with at least two players, blow the legs off five
zombies, and then have another player stand in one place while the
zombies crawl towards them. When they are all attacking the other play,
throw a frag grenade to kill them all and get the "Frag Master" achievement.