For future reference, the bedrock ceiling in the nether really helps. I usually try to find a gap in the bedrock ceiling and build an obsidian box around it.

For the fight I use fully enchanted nether it’s armour, a netherite sword with smite V, regen II and strength II potions. A shield helps too. Bring a bucket of milk.

For the fight, I summon the wither and use the shield to avoid taking damage from the blast. Then I repeatedly spam attack the wither which should bring it’s health down rapidly (I think they might have nerfed strength potions since I lost tried this though). Soon as the fight ends, drink the bucket of milk or the wither effect will kill you. You might want to try this strategy in creative first (creative for setup, survival to try it) to get a feel for it.

This has worked for me every time, even on hard.