Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: Sprint attack them, like creepers, or shoot it with arrows. You don't want to get the Wither status effect. Also, take advantage that wither skeletons are 2.5 blocks tall by putting a slab above their head so you can run freely but they can't, creating pockets of wither skeletons you can easily pick off one by one.