[Taqwus]

Wrath of God, Arcane Nexus, Strands of Arcane Power, Fata Morgana and Forge of the Ancients come to mind. If you've got the magic, might as well use it. Plus, the AI doesn't seem to good at shutting down globals for some reason; perhaps it prefers empowering commanders?

Also, summoned troops and forged equipment... against Vanheim, you'll face lightning attacks, so don't forget lightning-immune troops (notably Mechanical Men and Iron Dragons in addition to the air-type units such as queens, elementals and sylphs) and Rings o' Tamed Lightning.

If you want to try something impractical, wacky but possibly still fun, try something oddball like a Gift of Reasoned Gargoyle with a Snake Bladder Stick, Charcoal Shield or Lead Shield or Luck Shield, Spirit Helm (for the MR), perhaps Bone Armor or Rime Hauberk, Boots of Quickness, Ring of Tamed Lightning, and either a luck or antimagic item. Since the tactical AI isn't too good about units not running into clouds of poison gas, and Vanheim isn't inherently huge in nature magic IIRC, these should do some damage... bane lords are more cost-efficient, I'd think, but this is a game, not a retirement account. Iron Dragons have certain similar properties regarding speed, immunity and 0 encumberance, but they're far less equippable.

Tartarians are another obvious path. Best used with Gift of Health so their afflictions get healed, and devote a Fairy Queen to GoReasoning them. A human will probably be able to counter even Tartarians reasonably well if the game's taken that long and it dominates the map that much, but the AI will have trouble... so long as you cover the lightning immunity. Some Tartarians don't need that much additional gear to be nasty, although the earth-4 Tartarians get all slots in addition to high hp and other stats so they can be rather obscenely decked out. Other Tartarians can heavily supplement your mages, and some might be brutal communion leaders given a Crystal Matrix.

More troops can be had from Legion of Wights, as well. Ghost Riders is efficient against opponents that don't build fortresses, or for pushing back besiegers; and building a few Stone Spheres will let you systematically recon the map from a safe location. At that point you can do something evil like assign several casters to monthly cast Earth Attack or Seeking Arrow on the enemy's capital.