A very easy target: “His arguments were so simple, she was able to knock them down like sitting ducks.” The term comes from hunting, where it is much easier to hit ducks when they are sitting on the water than when they are in flight.
sitting duck definition
An easy target, as in If you park in front of a fire hydrant, you're a sitting duck for a ticket. This term alludes to the ease with which a hunter can shoot a duck that remains in one spot, in contrast to one in flight. [First half of 1900s]