Use the Chainsaw With Any Weapon.

This one is actually pretty funny to see.
This is also an easy one to accomplish. To do this you need to knock down your opponent. Make sure you have your pistol out. Go up next to them and slide your finger from the A button to the B button on the controller. If you did it correctly, they will still be in the downed animation (although they will have died on their screen), and you will be carrying them on your arm.

Unfortunately, I was unable to move doing this, however their body does act as a sort of shield as well. I'm not sure if this really gives an advantage as I was unable to move at all, except aim my gun, I couldn't even turn. However I heard it is possible to move, but I will have to research this one more.