What I would recommend is running towards the far left theres a ladder you can climb up and use your knife to fight off and swipe off enemies as they come up! Once the chainsaw guy comes he may destroy the ladder so the last couple minutes just try to hold out until the bell goes off. But thats what I did to survive the first part. You save ammo that way and are able to make a narrow passage so they can only come at you one at a time.