In the forest level, when you come across the river, don't cross it right away.
Instead, turn around, and head to a small nook to the west (see image, follow 
the arrows). Proceed until you come across a car - there is a chainsaw nearby.
Unfortunately, it's out of fuel. 

When you get to the farm, go into the basement and find the generator. Use it
you'll be asked whether you want to turn on the power or fill the chainsaw. 
Choose the latter. With the filled chainsaw, go to the large shed in the north
western corner of the farm. The door to the shed is boarded; cut the boards 
with the chainsaw. Open the door, go inside and kill the boss. You'll get a 
rather good ending.