This system will most likely only apply to the Mage. This should be separate from the Mage's regular ranged attacks, which involve one key press, and which travel independently in a straight line (likely a fireball)
The magic targeting system, likely initiated by the 'S' key, would work like this:
When the key is down, send out some sort of cross-hair-like graphic in a straight line. Put some acceleration on it so it has some sort of "casting time". Starts slow, and gets faster the further it travels.
Continue to move the cross-hair in a straight line until either the key is released or until it hits a wall/goes off-screen. If there is a continuous acceleration on the cross-hair graphic, then if you try to hit something that is very far away, the cross-hair will be moving pretty fast by the time it reaches your target, thus making it more difficult to hit with the attack.
When the key is released, stop moving the cross-hair graphic, and initiate the spell and its animation (likely a lightning bolt). The collision between the spell and the targeted enemy/object should not not occur until their actual pixels come into contact. This will make it so you need to lead the target if it's moving. Not to mention, it'd be pretty goofy to have the enemy die/disappear before the bolt even touches him.
The amount of mana this uses up as compared to the other attacks will probably be better determined once we see it in action.