Ive taken down a few out buildings and barns here . the tool I use the most is a matabo and cutting disk. Harbor freight sells a nice 7 inch tool for about 40 bucks and a 10 pack of blades run about 5 dollars. And it makes a clean cut , very thin without tearing up the wood. Slices thrugh nails like soft butter.
http://www.harborfreight.com/7-inch-angle-grinder-67648.html
http://www.harborfreight.com/pack-of-10-7-inch-cut-off-wheels-for-metal-47576.html