I beat the game with every god, using only the spells of that god, except Persephone (my Persephone game is on the mission right before the Crucible I think, I was about to finish it and the triggers screwed up, leaving me stuck at a cutscene - hmmm, maybe it was because I tracked down marduk and jadugar and slaughtered them, using my group of 12 dragons

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Stratos was definitely the hardest, James the easiest (Sirocco!) Once you convert all of his initial souls though, it's not bad, since cloudkill refreshes so fast (still harder than James though, since you cant use all Bovine Intervention/Bore to stop him from EVER getting any souls back, but I digress.)
For the first rush, guardian a couple flurries and get a yeti to help out with defense. When he starts to get close, stop him with a fence - he attacks from the right hand side, so you can make the fence touch the edge of the island and force him to walk around it. Then toss in a Cloudkill to kill his units while he is walking past. Don't be shy about using Lightning/Chain Lightning/Frozen Ground while you're at it. With a little luck, his units will die and the flurries will help to keep him pinned for a bit. You should now have time to do whatever you want to - you can get your army more organized after his initial rush, or try to convert souls, or whatever else you think you might be able to do to help out with the next time he attacks. If you can kill him, great - but don't count on it. He has a ton of HP, almost always keeps his shield up (Silverbacks kinda suck in this level because of it) and is VERY quick on the heal spell.
His future attacks will be worse and worse coordinated and balanced, up to a point. Expect lots of locusts, and keep a chain lightning ready for this occurance. It works wonders.
Eventually, you should be able to convert all of the souls from his initial army, except for possibly the ones he uses for manahoars. Dont forget that he can't shield the manahoars, so freeze them when he goes to run away to try and grab some freebie souls.
With his army defeated, all you have to do is walk up to the megalith to trigger the cutscene, then walk around and systematically take out his manaliths - use cloudkill on the MANA FOUNTAIN, not the manalith itself, and you should be able to toast the creatures easilly. Be very aggressive when he attacks you while you are taking out his manaliths - dont let him get the souls from the dead guardians if you can help it.
Remember that there's no reason to be shy with Soul Wind, since he can grab the souls anyway (allthough you must try to keep it away from him anyway), and don't hesitate to "make another stand" if he gets too many souls back from his dead guardians.