My preferred method: Grab a couple buckets of milk (or milk bottles if MFR is present) on your hotbar, then make a standard 1x2 tunnel about 64 blocks long. There shouldn't be any caves or other tunnels nearby, but otherwise it can be basically anywhere. Now at the end dig out space to place the blocks for spawning the wither; the middle soul sand block should be right in front of your face. Spawn the wither, move back to avoid its explosion, then head back in with a sword. Just keep whacking it at maximum sword range, backing up as it breaks blocks. It can't hit you with its projectiles in this confined environment, and if there's no space other than your tunnel it won't move to change that. You may occasionally get a wither effect, just drink some of the milk; make sure that you save the last bucket/bottle for after the fight is done. Best part of this strategy this can be done with purely vanilla mechanics and just iron.