>If possible, get the first Survival Guide
>only open 1 or 2 rooms (Nell's and storage), and buy the ammo finder (saves time). Enough ammo is generated outside of the rooms that it's not going to be a huge deal.
> upgrade handgun to max firepower over ~6 waves. You should have more than enough shots.
> save for Nell Magnum
> Nell fire rate up
> Nell damage up
> Let Bell do most of the work while you just kill fast zombies

On subsequent playthroughs, after you have all Survival Guides, just put it all on Nell and let her do the survival run.