In my opinion, the most versatile and useful art is Titan's Roar. It deals decent damage (600) and doesn't consume mana; and if you also upgrade air magic, you can get an incredibly powerful effect.

The impact force of Titan's Roar does not depend on upgrades. It will always be 600, but the Sphere that enhances air magic will give an additional 50%, and the damage will be 900. It won't be possible to get more than that. Also, although lightning doesn't consume mana, it still takes a turn to cast. And in prolonged tactical battles, there will always be something more effective than lightning. Mass slowing, acceleration, cloning, or elementals to reduce recoil. And, of course, removing curses. However, when a warrior has no mana left, it can be very helpful.

By the way, the sphere will give a greater bonus to a powerful mage, and a regular lightning spell will deal more damage than Titan's Roar.