Thanks 2hs. My build is 5 sorcery, 1 life or nature, plus warlord, artificer, conjurer, alchemy. Similar to yours, I think. My strategy is to get Malleus and another hero or two, make them items with guardian wind, magic immunity, flight, and some + attack and to hit. I send them out to tear up anything in between me and victory. It has worked well but its weaknesses are 1. invisible enemies, and 2. spell blast, which prevents me getting my guys equipped or even summoned.
I personally wouldn't have bothered with Conjurer; I'd rather invest in a second Life book (to guarantee Healing) or Nature (for the incredibly useful Web). Another option is Archmage, which increases your spell skill by a lot, and more quickly.
Other than that, this is a build that's tailor-made for Halflings or perhaps High Elves: you can have powerful ranged defenders pretty much out of the gate. Don't forget to give them magical and/or melee-based backup, though; too many enemy units have ranged immunity or are likely to know a spell like Warp Wood.
The best way to counter Spell Blast is:
1) increase your spell skill, so as to cast spells in fewer turns; if Create Artifact is taking 20 rounds, that's 20 chances for the AI to decide to do something about it.
2) If blasted, immediately restart the spell. Enemy wizards have ADHD, and don't usually throw the same spell again for a while.
3) Have at least one global enchantment running; that way they'll try to cast Disjunction rather than Spell Blast. A good one is Detect Magic, since it can help with #5 below.
4) As I mentioned earlier, they can't blast you if they're busy casting the Spell of Return.
5) One last cheat, which you may or may not consider ethical: save skim every turn or two, and if blasted reload and replay the round. Enemy wizards will often "change their mind" (i.e. roll a different random number). However, if you have the Insecticide patch, this exploit may no longer work.
As for invisibility, short of an item with True Sight the best defense is a good offense; make sure at least one of your heroes is good at melee combat. If they have Thrown, so much the better; build 'em the best axe mana can buy, and let them chop those nightblades and air elementals down right where they stand.
Another alternative is to conjure up some Sky Drakes to accompany your heroes; with five Sorcery books, you have a better than average chance of researching or otherwise acquiring that spell. Of course, that can become a Spell Blast issue as well, so proceed at your own risk.
If I think of any more tricks, I'll add them.