MoM is a great game, and I hope Stardock makes a good sequel after all.
If you Google around enough you will find a lot of valuable information on how combat works, bugs in the game, etc.
The cheapest winning strategy is 11-Death books, pick Wraiths as your Rare spell, select High Elves, then cast Wraiths IMMEDIATELY. Pump everything you have into Mana production (liquify your starting gold into mana, keep converting gold to mana as you get more of it), because that early on, the bottleneck is NOT Casting Skill but Mana production. Summon ANOTHER group of wraiths as soon as you finish summoning the first; life-stealing wraiths are almost indestructible early in the game (and if they get injured, just eat some low-resistance units to get healthy), and you can usually win the game by 1405. As someone else posted, wraiths also leave behind hordes of undead garrisons for newly-conquered cities, making 11-Death even faster and stronger than the popular 11-Life/Torin the Chosen strategy. This strategy also works well with Life (Torin); it doesn't work that well with Chaos (Chaos Spawn), Sorcery (Storm Giant), or Nature (Gorgons) because those units are weaker in many ways (slower, can't life-steal so you may need to rest them, don't leave undead armies in their wake, and unlike Torin they can't gain experience and items, so you can't get a superboosted Torin-with-Heroism-cast-on-him effect).
The typical starting game at Impossible: liquify gold into mana and summon Magic Spirit as fast as you can; use it to scout around the map. At the same time, keep your starting troops at home and increase taxes as much as you can without having rebels. Like with every other city you will ever build up, build in this order: Builder's Hall, Granary, Smithy, Marketplace, Farmer's Market (after that, it depends.. probably a shrine if you need it, else a sawmill + forester's guild as a precursor to miner's guild + mechanician's guild).
Everybody has their favorite normal unit, apparently. I am firmly in the Paladins camp because Paladins are so efficient (just place them so they always attack, to gain the First Strike benefit, using Flight if necessary). Champion Paladins with adamantium and Holy Arms in the background deal 48 swords at 70% accuracy + armor piercing, and get 8 hearts per figure or 32 hearts for the unit--more than almost anything not named Behemoth. Paladins really only have three weaknesses: they can't fly, they can get picked off by slingers, and Pikemen severely injure them. Flight and Guardian wind guard against Cracks Call and some nasty halflings. Champion-level experience, Charm of Life, Adamantium weapons, and Prayer or High Prayer means that even Pikemen can't severely damage Paladins, though one must be careful around them. (One satisfactory solution is to cast Call Lightning or some other combat spell to kill pikemen while your paladins run around dodging attacks.)
Mortu has my vote for toughest hero, even though I rarely play Death Magic and don't get to use him (Roland is an okay substitute, but has lesser hit points). With proper spells (Iron Skin, Regeneration, Flight, True Sight, Spell Lock, etc.) and items, even an inexperienced Mortu can kill anything in one hit and not have to suffer a counterattack. And with experience or at least a lot of spells/items, he can even take Sky Drake and Great Drake attacks and suffer only a few hit points' damage. This means that you can have Mortu single-handedly taking out tough nodes, enemy fortresses, etc. while your other heroes band together and wipe out the other half of the world, retiring to your fortress to boost your overland casting ability after they get Demi-God status.
As for spell combination, I think an Impossible game I just finished had the best combo: I started off with 4 Life 3 Sorcery Myrran and Warlord, the Myrran part to get Trolls (paladins take too long to get; the idea was to use War Trolls to kill enemy mages quickly so I would have Myrror all to myself). I eventually won 3 Nature books, 1 Chaos book, Node Mastery, and Archmage (making it easier for me to get spells through Counter Magic, and my own spells are twice as hard to dispel, and best of all, +50% bonus to casting skill improvement--very crucial, since casting skill is the main bottleneck in mid-game when you have no mana problems due to Alchemy and gold-producing cities). I also conquered and bartered spells a lot so that most of my spell discoveries in lairs/nodes were spells that nobody else had.
I also had a lot of luck in that I killed my Myrror competitor, Raven, very quickly with a swarm of War Trolls that systematically took over his Dark Elf empire one city at a time. Then I was even luckier when Merlin broke through to Myrror (boo) with High Men and build several big cities next to adamantium mines (yay). The computer develops cities quickly on Impossible, but by the time Merlin cranked out adamantium paladins, I had 43 fame and thus could (and did) summon all 6 high-level heroes (including Torin) and took over his paladin factories. I continued to settle Myrror with as many Dark Elves as possible, because they generate so much mana just by breathing, PLUS they can make most of the advanced structures including Merchant's Guild.
This is why this combo is just about perfect: you get Stream of Life AND Gaea's Blessing (cast them on all your cities, jack up tax rate to maximum, and thanks to Alchemy you will never worry about gold or mana again). Dark Elf cities generate a ton of mana and gold. Warlord + Crusade + Altar of Battle made my two adamantium paladin cities priceless, and I eventually got almost all of the best spells (too bad you can't have both Life and Death, because I love some of the Death spells like Black Prayer):
Flight (gets rid of paladin's main weakness: Cracks Call), True Sight, Invisibility, Magic Immunity, Guardian Wind (no slingers worries!), Spell Lock, Incarnation, Charm of Life, Holy Arms, Crusade, Altar of Battle, Prayer, High Prayer (for paladins and heroes). Also useful are Planar Travel and Plane Shift.
Web, Call Lightning, Regeneration, Iron Skin, Counter Magic (combined with Spell Lock = your heroes have nothing to be afraid of when assaulting enemy wizards' troops), Dispel Magic True, Haste, Dispel Evil, Healing, Mass Healing, Cracks Call, Confusion, Mind Storm, Banish, Creature Binding, Phantom Beast, Phantom Warriors, Air Elemental, Ice Bolt, Psionic Blast (for general combat)
Gaea's Blessing, Stream of Life, Raise Volcano, Change Terrain, Transmute (terraforming/city development--you can even settle what was once the North Pole or South Pole by casting enough Volcano + Change Terrain spells)
Suppress Magic, Time Stop, and the nifty Spell Blast (along with Spell Lock and maybe Magic Immunity, this spell is in a class by itself). Nature's Awareness helped me keep tabs on everybody better than Awareness by itself. There are also some other nifty spells like Earth gate, Wind Mastery, and Just Cause (which not only increases fame by 10, reducing army upkeep costs, but also reduces unrest!) that are not killer spells but are very handy.
By the end, I had every hero except Roland parked in my fortress to help me cast Raise Volcano + Change Terrain faster. Roland was a one-man wrecking crew with Regeneration, Invulnerability, Planar Travel, Iron Skin, and a lot of other spells cast on him; Sky Drakes/Great Drakes did zero or one point of damage against him, and since he can't cast spells anyway, I didn't feel bad having him roaming around.
I can't remember any MoM game I've ever played where I had such a complete package of the best spells; I came close a few times, but I think this was the first time I had all of the great Sorcery/Life spells combined with the Raise Volcano/Change Terrain combo--too bad the game is bugged so volcanoes don't leave behind mineral deposits, ever, but hey, that's what MoM2 is for.
