I'd like to start off by saying this might not be the BEST way to get this, but its a solution none-the-less, as it worked for me.
Get another 3 people to help you out (would help if they're good and familiar with the game)
the start doesn't really matter, just kill the zombies and rack up as many kills as you can, as there' will be another 3 people it's up to you which window you pick. i chose the window at the top of the blocked off stairs.
mid level 4 someone should open the back part, not the stairs. go and get the trench gun, ($1,200) and some bouncing betty's.
go into the little room under the stairs (all of you!) and blast away the devil dogs, stay there aslong as you can untill you feel you can survive while running outside. using the random box whenever possible, as it's not points your after.
go into the main garden bit (with the spinning trap), through there and go, all of you, and go to the shed, have 2 people wait at the main door at all times and get the juggernaut perk.
- each round put betty's near windows/open spaces to make sure none creep up without you knowing
- stick together so you can be quickly revived, not losing guns
- ALWAYS have juggernaut perk active (speed cola and fast reloads is up to you)
- trench gun, ray gun and a heavy machine gun are the best guns to use (i had trench and ray gun (got ray gun after using the F ____ machine gun)
- stay out of the open (under stairs is the safest place when not chasing after random box