Immediately after passing the minefield (see Hare Triggered achievement), Ben will be a captive and ready to be quartered. This is pretty straight forward. If you mention anything about Malcolm Corley, the bikes will tug on your arms and legs. Instead, use the "Let me go or else" option. Then use the "I'll call you names" response. At the next option select "Diapered Dynamo". This is the nickname Malcolm used with his dying words. Once you say it, Mo will stop the torture and the achievement will pop.