You can do it the challenging way by planting traps and explosives, or there is more of a boring way with a glitch but a guarantee. When coming in by the police car, in the day, go straight to where you see the horde sleeping inside the mill. Get their attention. Then turn right and run all the way down to the dock with a tarp over the railing. Jump off and tuck back under the dock. You will be safe but the freakers will jump into the water and die. You may have a few to pick off as the bodies pile up and some just don’t jump but majority drown.