As the Witch's lawyer, I would point out that the outcome of the competition was to discover the best PERSON, not the best POISON.
Otherwise, wouldn't the Cruel Tyrant be really really upset to find that both competitors had brought Water to the competition?
In fact, this result also yields the best POISON, as the Witch may know she can do better than the Wizard, but does she have a numerical measure? Would she make her own antidote anything less than her very strongest poison? The C. T. should be glad to see what the Witch uses when she wants to save her own hide.
However, as a humble Denizen, I'm eager to solve the problem you had in mind. Does your comment imply that my solution is not a winner?