But who will fight in the ARENA?
Will Natasha do it HERSELF, or will she turn on AUTO-BATTLE?
After all, the main key to my victory is my TACTICS with my ARMY (i.e., the manipulation of these 2 components).

No auto-battle!!!
Her task is to choose the best tactics for each side. If the arena is roughly equal, it is sent to the opponents so they can try to figure out the best way to play themselves.
Sometimes even very strong judges overlook something.
But here, unlike the Heroes League, there is a higher probability of a draw, so that no one feels slighted.
However, I doubt any particular tactics will be applicable here. I think it'll just be a slugfest without magic... ;)