This is just really easy in a local custom game with 2 controllers... should have this by round 3 or 4 without problems.
With your real player, buy the M14 off the wall so you have some better protection as your starting gun runs out ammo quick, with 2 players the first 115 Generator costs 400 points so immediatly buy that off the start with your guest, let those zombie guards down your guest and then when it's charged, on the left (under the challenge board) a chest opens and it will near always (9/10 times) have zombie blood in it as a reward for the Generator.
Pick up zombie blood and let the normal zombies down your guest and revive him, let him go down again and revive... for 3 times.
You don't need your guest anymore now.
Open the 750 door in the small room with the spade and go to the next Generator (above the AKu wall), just keep killing zombies there and you should get a zombie blood again either the first or 2nd drop. Active the Gen and just wait it out :) The achievement should pop right when zombie blood runs out. Enjoy your fairly easy 15 Gamerscore!