you can get the chainsaw after beating the first boss in the dlc. i believe it cost 50 coins to buy and 30 to refill its gas. You will get this achievement easily if you use the chainsaw alot during play time. I didnt use it much the first time threw so if u want to grind it out quick i would recommend using the chainsaw in the first torment room that appears after entering and exiting the first save area that appears behind the door to left left of the painting in the house.
also believe it has to be one playthrough. super easy tho so shouldnt matter