You'll need to have the singleplayer mode purchased for this simple method. If you bought the disc version then you have this mode.
From the main menu go to "SINGLE PLAYER" and then "SOLO SURVIVAL".
Only one map is unlocked by default, so select that one.
Once in the game just rush from waypoint to waypoint and try to avoid killing zombies. If they're walking aimlessly then ignore them, only if they run towards you. Basically just stand inside the bubble and wait.
You'll need to do this for about five minutes and then you should get to wave 8 which should spawn in the frag grenade.
Once you have the frag grenade by clicking in the right stick
you then want to throw out three Distractors with
, don't put them right on top of each other but have them slightly spread out and once you think you have a big enough cluster of zombies, throw the frag with
If you mess up and don't get enough then you'll have to wait out the wave till the next one and your stuff should replenish once the next bubble pops up.
If you only have the digital multiplayer version, this might also be possible with two people in a private match. You'll need to play some matches first in order to rack up the money to purchase the "FRAG" grenade for $10,000. You purchase things and setup your custom class under the "HIDEOUT" and then the "CUSTOMIZE LOADOUT" options from the main menu.
With the grenade unlocked and set to a class, load up a 1 v 1 on any map in preferably "KING OF THE HILL".
Have the person going for it stand in the hill and then have the second player throw "PAM" grenades (I believe you spawn with two with any default classes that have them) at zombies away from the hill, this causes a colored cloud to pop on their heads and attack the opposite team. Person on hill throw down your Distractors once those cloud zombies pop up and then hopefully those combined with whatever was already by the hill should total ten zombies so lob your grenade at them.