Currently you should cast the spell as a strategic spell immediately before the battle and then dispel it immediately afterwards. This way you wont waste a turn casting, and the cost will be the same as casting in the tactical battle. Actually less if you happen to be Ironeer. Does not make sense to me.
I agree with this being silly, I do like the ability to cast it in tactical combat, that makes it available for champs in the defense, or champions that doesn't see alot of action to also have superior armor.
I do not think unit enchantments should go up beyond 1-2 mana, compared to city enchantments, which I think are the ones needing a more dire whack with a bat on the mana on upkeep, since Usually spamming city buffs lets you research faster, and build faster, etc.