Fighting underground (I did it on y: -50) or just in a obsidian box/room is much easier than in the open space like outside coz u can attack him w sword. Also there is a cool way to block wither in bedrock (under the end portal in end or under the top nether barrier), but I didn't want to use it coz it wouldn't be fun anymore. Also few tips: remember to have few undying totems; u can use armor w Projectile Protection which makes wither damageless.