1. The Wither cannot destroy blocks warded with Thaumcrafts wand focus: warding.
2. The wither effect inflicted by being close to a detonation of one of its ranged attacks is much stronger than the wither effect from wither skeletons, so the mitigation that Protection IV grants is useless. As such, the one potion you absolutely must have to kill the wither is regeneration. Everything else is optional.
3. Fighting the wither in the open is suicide. In the second half of the fight it rushes right up to you putting you at zero range to try and dodge its attacks. As its attacks blow up the terrain you are going to quickly end up in a deep hole.
4. There are two halves to the fight. In the first half its easy to stay at ranged, and incidently the wither can be damaged by ranged weapons. Once it drops to 50% health you are in phase 2, where it is immune to ranged damage, and it additionally tries to get into your face.
5. The most effective way to fight the wither therefore is to prebuild a 1wide, two high tunnel. At bedrock, or in the side of a mountain. Start the fight by quaffing the potion of regeneration by shooting at the wither to draw it over. As it blasts the terrain back up into the tunnel to keep shielded from its attacks while shooting with a ranged weapon until phase 2. When it switches to phase 2 it will start to destroy terrain even more aggressively each time you hit it. Switch to a melee weapon and keep on attacking it while backing up into your stone tunnel. With thaumcrafts gear (sword of the zepher, enchanted thaumium armor) this fight has never lasted longer than a regeneration potion.