How does it work like if i have a {witchbane orb}
The short answer is it doesn't. In order to target a planeswalker with a damage spell that targets a player, you must target the player first, then declare that you are directing it at the planeswalker they control. If you can't target the player, you can't redirect it to their planeswalker.
You can still hit the planeswalker with spells that target planeswalkers directly (like {dreadbore} or {magmaquake}) and spells that don't target (like {earthquake}).
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