Diplomacy might work, and Izra with Vidomina, of course, but relying on that... Even on HL+U, against an opponent in the mid-game on level 11, neutral creatures might start joining him, and they might not stop until he conquers half the map with a Garrison of Order in each of his 7 stacks. It's quite difficult to level up mages; the hope is that one of the 5+ scouts will get a level-up in the early game. Although all probabilities are equal at 50%...
What I mean is - you can win against a diplomat, but relying on luck doesn't seem fair for a tournament; experienced players always avoid that, as far as I know. For example, choosing Luna against Sair on an L-shaped map is still reasonable.