Since no arena is specified I assume it to be the only logical place.

In space! (the winner is brought back to their natural habitat)

Everything else would be unfair to at least one of the parties.

I think the ants would outlast all the animals and insects. They might freeze solid and (some) might survive the unfreezing after the figh. Or they could form a giant ball where the ones in the middle survive. And who cares if 19kg die.

On the other hand, the oak would probalby last weeks and could sprout new leaves and roots again if given proper treatment back on earth.

So … GO TREE!

They asked me how well I understood theoretical physics.

I said, I had a theoretical degree in physics.

Yes but “the biggest problem” is most certainly not the lack of insurance for the daughter. Its a untrained child playing with the live of a patient.

If somemebody drills a hole into my skull I dont give a shit about their insurance.

Insurance protects them not me. This is absolutely about the doctor putting the patient into a huge unnecessary risk without the patients consent.