I completed this on Case 2, might not be possible on Case 1 since it looks like the encounters are pre-programmed during this case.
Immediately upon starting Case 2 (or really, anytime after), turn around and head back into the mall and make your way back into the Willamette mall safehouse. From here, you're going to exit out the double doors/garage on the main safehouse level (to where the tables with the food are) and head hard left (like they forced you to do on Case 1). Go through the areas with the busted cars to a set of double doors. Opening the doors leads you to a set of escalators. Walk up these to the main mall floor.
Right up here a survivor will often spawn with a modest amount of zombies around them, which should be easily dispatched without trouble -- assuming you has a decent combo weapon.
Once complete (or if there wasn't a survivor here), head back down the stairs and back into the safehouse. Talk to a vendor to ensure it registered you in the safehouse (may not be necessary as you just might have to enter the safehouse, but I did this as it seemed to fix any registering issues). Doing this will reset the area where the survivor can spawn, so head out to the same place as listed above to see if the survivor indeed spawned. If so, great! Kill and get another survivor added to the count. If not, just head back to the safehouse, talk to a vendor, and repeat.
Doing this should quickly max out your Willimette mall safehouse survivor count and get you the achievement (as well as unlock all the maps on the location vendor).