My tatic with the wither is, when it's health is above half, take out a bow, shoot it, retreat and charge back up. Rinse and repeat. This way, there's less of a chance that it will inflict the wither II status, and it won't be able to eat through the ceiling above you because it's busy tracking you. (it still does eat it sometimes though) After it's under half, blitz it with whatever sword you've got, and ignore that wither status, just slash it. We had it stuck in a stop where it was locked to me but couldn't reach me, so I had a lot of room to hack it while it tried to kill me. I was using a Potion of Strength (8:00) along with a Power 1 bow, which dealt considerable damage against it, and then the sword battle at the end of the first one, it lost half it's health in like 15 seconds. Potion of Regeneration would be very useful to bring along to counter the wither effect, along with a full hunger meter, and some Potion of Instant Healings too in case your health gets that low. Also, don't use any Potions of Invisibility, it sees right through it. Diamond armor with some projectile protection enchants will also be a huge help, but really only useful for the initial blast of the wither skulls, not the wither effect itself. And have a good base to retreat to if it gets too rough, somewhere you can hide for a few moments before it finds you.
Also, I never died once to any of the 3 withers. :V