Use the Chainsaw With Any Weapon.

This is also probably the easiest to accomplish, as all you have to do is when you see someone running at you with a chainsaw, first make sure your lancer is out and you are facing their direction. Then when they are right next to you, change weapons. Of course this doesn't allow you to use the chainsaw after the dual, but it is a neat thing to see a Torque Bow ripping through someone.