In Unfinity, one of the core mechanics is hats matter, and thats honestly just adorable
For one, spells and abilities cant be countered, but the other means damage cant be prevented
However, if you do choose a target, and that target is illegal at the time Witch's Mark tries to resolve, the spell won't resolve and none of its effects will happen
In limited, drafting plenty of commons like Focused Funambulist and Bamboozling Beeble will help you scry to the cards you need and keep you swinging in with an army of fantastic flying fellows
Do you aim to win the turn it comes down to deny them its use, or just sacrifice it before your opponent untaps
While this is definitely true if youre trying to play it in Commander, it does enable numerous devastating combos