Discussion of the Academy faction (Mages and Wizards). Tactics and their specific features.

For version 3.0:
1. Banish deals damage to all summoned units (including demons summoned by "Gate Opening"). The damage is equal to 10*(Hero Level + Summoning Magic Level), where the Summoning Magic Level is -1 for basic, 2 for advanced, and 3 for expert skill. Thus, as I understand it, the damage from Banish for a level 20 hero with expert Summoning will be 230 units – this probably won't be enough to destroy a summoned phoenix, but it will likely remove phantoms from the map.
2. The choice between Exorcism and Banish is a matter of taste :). Will you have enough mana? How many units will you be able to gather for the battle? Will the opponent summon all their stacks as reinforcements or limit themselves to individual units? There are so many random events in the game! :) I would still prefer Exorcism, because you will have to use Chaos anyway, and in a battle against aggressive demons, there probably won't be time for both Chaos and activating Banish.
By the way, in 3.1, the Fire Lord perk doesn't work correctly (it seems to be a bug), so keep that in mind when leveling up :).