So I have a Legend character with the Vigilante ability track, which grants him a ride, a free set of magic armour, and a unique attack option, called a Brutal Kick.
Brutal KickEX: You jump high in the sky, then come streaking down towards your foe for a devastating kick, whether or not you usually use such attacks. As a standard action, you move up to your movement speed without provoking attacks of opportunity, and make a melee attack against an opponent in your [Melee] range, which deals additional damage equal to your level and can target a [Flying] opponent even if you are not [Flying]. You can choose to treat your Unarmed Strike natural weapon as a [Brutal 2], [Magnum] weapon for purposes of this attack.
When I created the character, I multiclassed into the Path of Destruction ability track, which grants the ability to hit more creatures in a round.
WhirlwindEX: If you would make one or more melee attacks during your standard action, you may add the [Volley] descriptor to a single one of those attacks; this attack with the [Volley] descriptor is applied against every opponent in [Melee] range.
Because this is key to the question, here is the text on [Volley] effects.
[Volley] effects expand the breadth and range of attacks. An attack with the [Volley] descriptor may be applied against multiple creatures: A single attack roll is made, and the result is compared to the AC of each affected creature as though those creatures were attacked individually. Abilities and effects other than the attack’s normal damage that take place as the result of a hit or critical hit only activate once as a result of the attack, and apply to no more than one creature the attack hit, of the attacker’s choice. An attack with the [Volley] descriptor may not be applied against a given creature more than once per attack.
What happens when I use Whirlwind on my Brutal Kick? Specifically, can I make the melee attack against every [Flying] and non-[Flying] creature in my [Melee] range; or only up to one [Flying] enemy, with the rest having to be legal targets for a regular melee attack; and does the bonus damage from Brutal Kick apply to all the creatures hit with the Whirlwind?
EDIT: Here is the text on [Flying], for reference.
[Flying] creatures are far enough above the ground that grounded creatures treat them as outside their [Melee] range (and vice versa), but at low enough altitude that they can be targeted by ranged weapons and are caught in area effects.