If you have diamond armor with Protection IV and some healing/regeneration of some sort, then just dig a cave with a low ceiling and spawn the Wither in there and whack it to death with a sword (with either Sharpness or Smite at a high level.)
Quaff an 8-minute potion of Night Vision beforehand because all your torches will be blown away.
Parts of your cave roof and maybe floor will be blown away too, of course - but the Wither will still remain within sword-reach. Most of the Wither blasts can't even destroy cobblestone. No need for an obsidian cave.
You can make it even easier by making a long thin corridor leading to the Wither cave-room and then nail it with a bow as it blasts its way to you, saving you from any damage to yourself until halfway through, when you have to get to work with a sword.
At any rate, dealing with the Wither in the open air is a bit of a nightmare IMO!