DOOM II (Classic)

On the first level, instead of dropping down, turn around and follow the curve around to get the chainsaw. Now you can either play the game normally and using the chainsaw when appropriate, or you can make a save, murder everyone in the first level, and reload until you get the achievement. You really shouldn't need to grind, though. The chainsaw is a great ammo saver when the enemies are sparse and you should get this pretty quickly.

MAP 16, after you pick up the blue key run down to the invincibility pick up and start chain sawing your way to the achievement. You will need to rinse and repeat a few times to get it. Make sure to make a save at the beginning of the level.

Assuming you have the chainsaw already and you haven't gotten the achievement yet... I found a great spot to mop up the rest of the chainsaw kills. On Map 8: Tricks and Traps (Ultra-Violent difficulty) go into the door you spawned into the level looking at. You will be greeted by a bunch of "Pinky" demons on the platforms above you to the right. Walk close enough to them to activate the lift and run back into the cubby at the back of the room near where the rocket spawns. Keep back in the cubby so you can't get attacked from the side. Now rip and tear "Until it is done!" I liked this solution because you don't have to worry about enemies shooting at you and the Pinkies are easy to kill with the chainsaw. Each run will net you 18 kills on Ultra Violent difficulty. Rinse and repeat until the achievement pops.