How to easily kill the Wither?

If you are looking for a farming type of setup, you can create a chamber in the top of the Nether of 3 blocks tall with the roof being the bed rock layer. In the floor place a piston pointing up. Build the Wither on the piston and as he is spawning, activate the piston to push his head into the bed rock block. This keeps it from moving around and taking shots at you.

Here is a video that demos it with another

of SethBling putting it to use.

Here is a example of fighting in a tight enclosed space that isn't totally safe. Basically the space is only large enough to spawn it in and for you to stand and watch.