Air Mages are one of the four classes of mages of the Hand Brotherhood. They practice air magic and can be found throughout the Hand Brotherhood's Air Tower. Their magic is possibly the most dangerous of all four types of mages.
Air Mages, like the other mages, are weak by themselves, but the Air Elementals they summon are not. When a mage is alerted he summons an Air Elemental, Which is considerably dangerous.
A summoned air elemental will rush towards Garrett, and if it reaches him, it will self-destruct, inflicting damage. It can navigate around corners, fly over barriers, and move very fast. The elemental however seems to have difficulty navigating through darkness, like regular humans, and it will not explode if it cannot find Garrett.
A plus side is that the elemental is summoned only for around 12 seconds, so if Garrett hides in shadow he has a good chance of avoiding it if it doesn't accidentally bump into him.
The Air Mage does not in fact have any direct means of attack, and can damage Garrett only with elementals. Take advantage of his relatively low health and go for a kill from the shadows before he can retaliate. However if there is a lack of shadows nearby, be aware that the Air Mage can summon another elemental, even if the first one hasn't hit yet, and it will become difficult to dodge two or more elementals at the same time. If other mages or guards come to help, the situation might become grim even if Garrett tries to run away, so avoid alerting the mages.
The Air Mages are immune to knock-out gas. The Air Elementals won't explode upon another mage, but presumably the Air Mage is immune to their damage as well.