Used helthing poison, when wither have half helth,use sword and kill he
Wither fight time

have u beat the ender dragon?
if u have, go under the ender dragon's bedrock pyrimad thingy (the thing that has the beacon thingy after you beat the dragon) break the void/portal thingy with mushroom, place obsidian sideways beside the bedrock, spawn he wither with heads all facing the bedrock beacon thing, then it should suffocate if u do it right
but make sure to run away cuz it can still shoot head/skull thingys.
this only works on bedrock editon

how can u fight it in a cave?
the wither is actually a great miner, just go deep down and spawn it the stone will stop the skull things from hitting you but it might inflict wither efect
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Any tips for fighting wither?