Doom 3

The chainsaw can be found in the Service Tunnel of the Communications Transfer stage in Doom 3. After progressing far enough, you'll see many empty chainsaw boxes. That's your queue to enter the door on the left. After picking up a PDA in the room, you'll be attacked by a zombie with a chainsaw. Just keeping backing up and shooting him as he can do a lot of damage. Once you kill him you can pick up the chainsaw or get one in a box in one of the small rooms that opens up. Now with this groovy weapon, you can start spilling some blood. The chainsaw is a very powerful weapon, as it can kill most enemies in only a few seconds. The downside is that it has little range, leaving you open to attacks. Luckily in the same level you pick it up, there will be many normal zombies walking around and on lower difficulties, you can just tank hits and go for the kills against stronger demons. If you kill every enemy possible with it, you'll get the trophy in the same level you picked it up in.