Miasma is a good nation with which to try out the Astral Wyrm I wrote about in the guide to buffing thread. He has Swamp Survival, so he's immune to the nasty effects of your dominion, and he benefits from the research paths Boron suggested.
Take a Wyrm, Astral 3, dominion strength 6, and a castle. That leaves enough points for 6 positive scales. Distribute those as you like. I'd probably go with magic 3 and luck 3, but then I'm a bit weird, and enjoy playing with luck - perhaps more than its worth
On turn 1, recruit a Lizard King, and on turn 2 make him your prophet. Its always fun to have a prophet who can Smite
Meanwhile, have your Wyrm research Alteration 1. Once you've got that, your Wyrm is ready to go out and conquer, since he can cast Personal Luck. Then start researching Enchantment. That will eventually get you a few things. At ench 2, you can add Astral Weapon to the Wyrm's script, and at 3, Astral Shield. The big serpent is now a force to be reckoned with! The great thing is that your Marshmasters can now cast Animate Skeletons - one of the most annoyingly effective tactics in the game. I'd then reseach Alteration 2, so you can cast quickness before beginning the Skeleton spam. After that, you're on your own as far as research goes, althought getting Ivy Crowns and Vine Ogres is a solid strat - and don't forget that Miasma starts with Summon Bog Beast as its national spell - and that Vine Ogres are immune to poison.
As far as troops go, the combination of City Guard and Light Infanty you start with works pretty well. Put the City Guard out front to do the fighting, and the Light Inf. slightly behind and to one side set to fire closest, so they'll use their javelins. I myself can never resist buying a force of runners to be used as flankers set to attack rear
If you do it this way, you give up the magical diversity of a rainbow pretender, but you gain a pretender who can conquer on his own, and much better scales. Later in the game, you can even use you Wyrm as a Comminion Master, and a bunch of Shamen as Communion Slaves for some heavy-duty Astral casting. Note that the Shamen are very cost-effective researchers, since they're sacred, and you'll likely have a bunch of them around.
Hope this helps.