Well, I just found this thread, and finished reading it (wow, that took a while).
If it's not too late, I wanted to point out a viable, and possibly very effective tactic for the Ta'jinn against the Cressians. (iirc- the stempa were really into alchemy as one of their advantages)
So, the Ta'jinn buy/steal/take from the Stempa an alchemical secret: alchemists fire. This substance can be put in vials where it won't ignite, then, if thrown, breaks and lights things on fire. Also, it's very difficult to put out (as any proper self-respecting, burning jelly will be). In a flash of inspiration, the Ta'jinn develop a startlingly effective new tactic: bombers.
Handing out many (between 10-20) vials of the alchemists fire to each Godsblood, they send them out in groups to attack Cressian cities. Each group has 3 Godsblood riders with shortbows, and 2 with the alchemists fire ammunition. The 3 with shortbows help protect the "bombers" from both ground attack, and the Cressian giant eagles. As a plus, the giant eagles can't engage at range, and even then are still at a disadvantage due to inexperience fighting other aerial foes. Likewise the Cressian ground forces, being experienced at throwing spears/shooting bows at ground targets are pretty much out of luck against griffon riders hundreds of feet up. Then, when the griffon riders fly over a fairly major Cressian city, they dump all the alchemists fire vials and head back to the Ta'jinn camps. From the number of Griffons, I'd say that there could be two such "groups" of 5 Griffons with Godsblood riders, with enough left over to still hold up a good scouting force.
From the looks of Cressian society, they'd have a hard time fighting these fires. Firstly, they have a cultural aversion to fire, meaning they'd be much less experienced at dealing with it than the Jongans in the last round. Besides just this, their primary building material is... wood. Wood burns. Wood burns faster when there's a bunch of undergrowth in the area. Cressia has a bunch of undergrowth. Offhand, I'd say that the Cressians would be very quickly looking at major brushfires where a few of the biggest cities used to be. This would cause chaos amongst the Cressian forces fighting the Ta'jinn, and even worse effects would be visible within the populace, who now no longer know when they're safe, even in their homes. After all, any moment now, fire might fall from the sky, burning their city like the others.
Major advantage: Ta'jinn.
This success leads to the Ta'jinn focusing on improving and consolidating their control of the skies.