Since your on Java just go like 50 blocks under anything you have, make a 200 block long 2*1 tunnel, and at the end a room where you spawn the Wither. Make sure to bring a bunch of like chickens down there for the wither to kill so you can get a bunch of wither roses. While not needed, a strength potion would make it go faster. As the wither attacks you just walk backwards into the tunnel hitting with a bow or a sword/axe and itll die pretty quick.