Remove the piece of wood or surface from the area where it is nailed using a crow bar, if possible. Barbed nails are often used to attach things such as window sashes and roof eves. If you pry them away from the base surface, it is much easier to remove the nails.
Pull the head of the nail loose with pliers, if possible. Sometimes it may be difficult or impossible to pull the head loose. If you cannot pull the head loose, then you will have to take a different approach.
Put on safety goggles and ear protection. Cut the head off the nail with the reciprocating saw. If you cannot get behind the head, then cut the nail wherever you can fit the saw. You want to remove as many barbs from the nail as possible.
Place an awl onto the remaining nail piece. Drive the nail through the back side of the material by hammering the top of the awl until the nail falls out. If you cannot reach the back of the nail, then pull out the remaining nail pieces with pliers.