Well First I have to say this is pretty damn cool stuff. A few comments though:
With the number of epic level creatures you have on the defending force (and I assume you have a similar number on attacker side or this will be over fast), you have a conflict on the scale of a small scale nuclear exchange between two superpowers in my opinion. This seems to me to be a nearly cataclysmic battle IMO.
Anyways, depending on the magical might of the defenders, since you have a place that you can retreat to, I would do this:
1). Gather all my spellcasters up and have them summon massive amounts of elementals, demons, undead, etc....whatever types of "expendable" creatures you can come up with, and simply send wave after wave of these into the enemy ranks. Constant, pressuring attacks to force the enemy to put its complete attention your way.
2) support this attack with precision attacks on specific enemy targets, in this order:
a) enemy spellcasters
b) enemy air units
c) enemy supply and logistic structure
3) The idea would be to provoke the enemy into attacking your fortress. Without seeing the terrain on the map it is difficult to do this, but here is how I would set it up if possible:
a) set up defensive positions around the fortress. Areas that you can constantly fall back to, basically a tiered defense.
b) Utilizing magic, use Hallucianary terrain and mass inv spells to position your fast moving troops on the flanks, and attempt to draw the enemy into a "fire sack". If you can get them to fix on your fortress, then flank them with your fast movers, you should be able to unleash hell into the firesack and demolish them. The key would be to ensure that they lose their eyes and ears (magic and flight assests) in the beginning attacks. If they do not attack you, simply continue to harrass them with expendable magic assests (if you get in trouble you can always fall back to your fortress).
4) some other "harrassing" attacks would be:
a) plague on the enemy army
b) highly specialized "strike" teams eliminating key players in the enemies army (high lvl generals, healers, and wizards)
c) Massive amount of "terrain adjustment". IE, if the army is split into several camps, take a large strike team, and use magic to alter the terrain to isolate them, then destroy the smaller forces with your overwealming strike team.
d) Magic altering of the weather to make the enemy army suck wind (blizzard in teh middle of summer would pretty much suck)
Just a few ideas I had here at work
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TLG
