I'm sorry, it seems I've misunderstood something?
I thought you were talking about the current Unity version, but were you talking about the old GMS version (before Unity)?
Also yes, I have played throughout the entire game in Hard Mode, and the Wither boss took me 2 attempts, when it's on its 2nd phase, it's basically on the ground, so you can spam tesla star shard.
Again, I'm sorry if I misunderstood, I don't know if you were talking about the current latest version of the game's Wither boss or the old one