I got the mid-air attack at about staff skill lvl 6 or 7, same as the other weapon's mid-air skills.
It's a useful move even for people that don't use staves as it ALWAYS knocks down enemies that it hits and it hasa wide area of effect.
Follow it up with quick-hitting weapons or spells to keep the enemy down till they die.
A tip for a fellow staff user: if a type of staff seems to use a LOT of RP, try using a weaker staff fo the SAME ELEMENT for a while, if you lvl up that element's skill lvl it reduces the cost of its spells by a certain percentage.
Working on your staff skill pays off in the late parts of the game, "gods end magic" (the ultimte staff) can potentially do over 10k damage to every enemy in a wide range, for a low RP cost too.
Of the 3 special skills, the mid-air one is useful vs a few enemies(dragons especially) the charged one is pretty useless, but the one that fires strong magic is essential if you want to obliterate whole rooms.