Better than the arm Achievement
Better than the arm30
Kill 25 monsters with a chainsaw
The earliest time you can get a chainsaw is during the side quest 'Genre: Horror Slasher', from Dave in Makat Village, on top of a submerged trailer. The quest will bring you to a Dead Zone, with 6 zombies inside. The chainsaw is located at the end of the ally, where you turn off the generator.
The chainsaw damages easily, even when fully upgraded. Best thing to do is slightly tap to attack.
Also after the 'Militarized Zone' main quest. You will see a survivor, Nora, who needs help on top of one the towers. Kill the surrounding zombies and she will let you purchase Military equipment. One of those equipment is a chainsaw.