Though I have beaten ALL versions of the final mission for the Keepers as of today (JUST beat the Halfling-Lizard-Undead route; tragic ending :*( ), this is one of the biggest pet peeves of that mission. Unless I'm allied with the Humans during that mission, the unit in question is a SCOURGE on my units.

That Air spell that damages flying units I've tried and since it can get resisted, I've found that a lackluster option. The best I can think is to somehow be able to summon a powerful flying unit (namely one of the dragons from the highest Life and Death spells) or otherwise build several powerful flying units like Demons from the Undead or such, but I'd like to hear any other solutions you have.

Otherwise, now I can just focus on the Cult of Storms campaign. :)

Since the Air Galley has only an unenchantable ranged attack, the main counters to the Air Galley are units that can attack the Air Galley and have Physical Immunity. This includes:
- Air Elemental
- Undead Wraith with Wind Walking
- Incarnate with Wind Walking

As you can see, having at least three Air spheres is very useful against Air Galleys. Even if you don't have Wraiths or Incarnates, simply casting Wind Walking on powerful melee units (Earth Elemental, Karagh, ...) can do the trick. You can also cast Wind Walking (or Free Movement) on ships; two such enchanted Dragon Ships or a single Galleon will usually take down an Air Galley. However, battles involving only Ballista-type units can be very random. (Note that casting Wind Walking or Free Movement on ships is considered an exploit by some people.)

The next best option would be Level 4 flyers, i.e. any of the dragons or the Astra. These units can generally go toe-to-toe with Air Galleys but most of them aren't actually superior; if they are not supported with other units (Priests can be very useful) then they should be heavily enchanted. Level 3 flyers generally need 3:1 odds or 2:1 odds and heavy enchantments to come out on top.

Heroes with 10 DEF and a ranged attack can also work quite well.

For static defence, three Ballistae are pretty much guaranteed to take down an Air Galley at a loss of one Ballista. Two Ballistae and two Archers also work but you often lose both Ballistae. However, in terms of upkeep Ballistae and Archers aren't exactly cost-efficient compared to one Air Galley.

So in conclusion, other than flying units with Physical Immunity the Air Galley has no real counters. Just so you know, there's also the Halflings-Lizards-Elves route added in AoW+.

I don't think Wind Walking allows walkers to attack flyers? Just tested.

Was just about to respond to that. For what it's worth, I play the vanilla GoG version of the game, so I don't know if the spell was altered for modded versions or not. I don't trust a lot of mods with games other than those for games on Steam (ex: Skyrim) that are guaranteed to be safe. You don't know when you might get a virus from such or if it might make the game bug/glitch out or crash. =/

Okay, so Air Elemental it is, if I even have that many spheres in Air in all my campaigns.

BTW, for clarification, there is no difference in text/storyline for the High Men ending for the Keepers regardless of whether I go the Lizard or Dwarf route, correct?

Yes, the two paths are identical. The same goes for the two Undead paths in the Cult campaign, though in this case at least your starting cities in the Valley of Wonders are different. Furthermore, even though the campaign texts differ, the Highmen and Undead Valley of Wonders scenarios the same for both campaigns, which is why Elric and Talic don't appear.

Regarding Air Galleys, if your concern is only the last campaign scenario but you don't have heroes that can deal with Air Galleys directly, there's also the option of summoning Gold/Black Dragons. In the Valley of Wonders they are essentially cost-efficient counters at 2:1 odds because you can research all spells in earlier scenarios, so you will run a large mana surplus anyway. A hero with Spell Casting V takes 5 turns to summon a Dragon and generates enough mana on his own to support two of them, so if you crank out Dragons and keep them in stacks of two (or simply together with another reasonably strong flyer) then Air Galleys shouldn't pose much of a problem. This strategy really only works in the Valley of Wonders of course.

Or if you have too much gold you can simply build tons of Archers. They're not cost-efficient at all but at 8:1 odds they do get the job done.

Okay, so no sense doing the Lizard path for the Keepers in relation to High Men and the Cult of Storms for the Undead, then. Saves me precious time.

I have managed to summon at least one Gold Dragon in my initial playthrough for the Keepers-Dwarves-High Men finale. I prefer as far as Keepers, namely if I ultimately side with the High Men or Elves to go with 4 spheres in Life or Air, respectively. The former is immune to the Sacred Wrath spell, which makes combat against evil foes easier. Either my hero or Gabriel can cast that and if my other troops are High Men, they will take no damage while my enemies get taken down a notch. Elves, at least, I'd go with 4 spheres of Air and would instead be able to summon Air Elementals.

I've been tempted to replay the Keepers campaign but try out Fire or Earth spheres and see how that goes. That may make Air Galleys tougher to beat, though. =/

For the record, other flying units are not the issue. They at least can/will attack ground units in melee and can be destroyed. I found Red Dragons in particular to be easy pickings, especially against a unit immune to fire, like the Dwarf Firstborn. Take a few of those at max rank, if not a whole squad of them, and they will turn said Dragon into a prime rib dinner! ;)

Use a ruleset from heavengames if you want to play with Fire. Rulesets are fun in general but Fire spells are so overcosted, especially the fireball and other damage spells.
My own Sword and Sorcery makes those spells very cheap, but I think most of them do.
Or you can download DevEd and edit the release file yourself. Imagine a world in which you have to pay for Fireballs!!! Free fireballs for all!

While we're promoting our own mods, AoW+ also makes Fire spells stronger (and Air Galleys weaker and the campaigns better) but it also makes a lot of other subtle changes to the game and may not be everyone's cup of tea.

Confirmed after testing on "Earthquake": 3 max-rank Dwarf Firstborn are enough to take down TWO RED DRAGONS. One was left at about 1/3 to 1/2 of his health, but that's still quite remarkable, if you ask me. :O