Once you reach the part of the story where the bridge is taken out, you start at the spilled fertilizer. Get off your bike and grab some with your hands. Then travel to the Mine Road. One of the enemies you will run across is a female red haired biker with the chain saw. Immediately switch your weapon to the fertilizer and then Attack her with it. You will need to be pretty close. Ben will throw it in her eyes. She will crash and then you will get a very short scene of Ben grabbing the chainsaw. At which time the achievement pops.