The focus transfer mechanic should be removed from air shield. Every other shield in the game provides attunement and some significant boost with no drawback. The focus mechanic with air shield ends up being a hinderance in many cases. Many Air mages do not use it and lose out on the attunement boost that every other tree gets. The new air shield should have attunement plus some boost related to the Air tree. Chance to stun melee attackers, misdirect projectiles, anything really. Using focus as a shield is a good feature but it makes more sense to put it in the Focus tree.