Things I think I'm going to try: Sea King's Court, Pale Riders, Mechanical Men.. of course abominations if you have the research and the astral gems (I'm assuming no one does besides maybe Ryleh
)..
I don't know about Naiad Warriors, I imagine they are pretty decent..
There's always the Sea King's Goblet and the Barrel of Air to get your national troops underwater, too. And once you're underwater, you can recruit tritons. Probably the best bet, though, is to get thugs equipped with Rings of Water Breathing and magic resistance items and send them underwater by themselves or in small groups. I guess. I'm not really sure what we're going to do, lol.
Piercer is a crossbow that works underwater. Incinerate is a fire spell that works. Of course water magic would be the best if you've got it.
This is going to be a tough road to hoe.. but if we all get at least one or two underwater groups together, we can outnumber Ryleh and take some provinces, at least. Any strong army of his that you see, just run away from. Don't engage. He can't stop all of us at once. Once we whittle him down to a few provinces, we'd have to start winning some battles, admittedly. Hopefully we'll have a strategy together by that point to do so.
I still remember this one game I was in, where I won every single battle I was involved in and still lost the game, pretty early, too. I was under attack from Ermor in the north, and I moved to intercept his army. Defeated it. Then Pangea attacked me from the south and since I wasn't down there, he took over several of my provinces before I got there. Once I did, I defeated his army, but I was beat-up and had to retreat from his reinforcements. I went back to my capital and started summoning troops. He took the remainder of my provinces. I finally left my capital, won another battle, but got beaten up bad again. He had more reinforcements seige my capital. I broke seige, won again, but this time was down to only my god. He seiged me again and then dominion killed me.
That is the game that is most memorable to me, so frustrating because I won every single frickin' battle!!! But, if you are outnumbered, there is basically nothing you can do. I think we can take Ryleh down, just no one engage his armies if you can help it. The goal is to take provinces, not to fight.
Edit: I change my mind about over-casting Stellar Focus, he will just over-cast it over us! He's making stupid #'s of astral gems per turn. We just need to rush his punk a$$.