[Endoperez]

So, you want to be able to defend a choke point as well as you can, but still be able to attack?

You could go for superior defence and then either attack with your forces or just raid him so that Vanheim gets weaker and weaker. One good way to do this (vs AI) are the spells that kill population and/or conquer them. Also, you should try killing Vanheim's armies with spells like Flames from the Sky, Leprosy, Murdering Winter. Use MW in cold provinces and versus non-cold-immune or resistant troops, FftS versus units in moderate or warm provinces and Black Death if time is not needed or if you have Demiliches.

Wish is never cost-effective. Wishing for Doom Horrors devastates the AI, and they are almost so overpowering that ruin the game (single-player is too easy if you use Wish in cheasy ways). Wishing for a few Sphinxes and putting *them* to frontline will give you quite defending force (but as I said, is not cost-effective). Natarajas excel after they get some equipment, so they have to be Gift of Reasoned. However, against AI Wraith Sword (life drain) and a few other toys like Demon Whip (AoE fire), any length 4 or more weapon (repel) and Scultata Volturnus (shield, attacks with lightning) or Charcoal Shield (gives Fire Shield). Remember to add reincigorating items or he will quickly pass away, especially if alone and/or with Boots of Quickness/Jade Armor.

Have you already conquered the seas? They offer great mobility, but unfortunately Water summons tend to have upkeep, suck and/or cannot be summoned on land. Try some undead to get started, they can also attack on land. Also utilize your Arch Theurgs and astral spells like Astral Travel: give them Whip of Command and Crown of Leadership, and use misc. astral boosters like Crystal Coin.

Raid. Don't even try to hold the provinces. Take them, leave, take another. Unrest will go up with each battle, and you can raise the taxes to 200% during the turn you own it. Also utilize spells like Ghost Riders, Call of the Wild (especially if you have it for free through Soulstone of the Wolves) and Imprint Mind, possibly together.

Summon all high-level uniques you can. Air Queens, Earth Kings, Anthrax (Lord of Banefires), Ice Devils... Lots of magic, lots of physical strength and lots of possibilities.

If you have money problems, forge Alchemist's Stone and Alchemize. If you have gem problems, forge few boosters of every path and go through *all* your provinces with your mages casting Auspex, Dark Knowledge etc. site searching spells. If you want, you can also try to hoarding gem-generating items like Fever Talisman, or unit-generating Soul Contract/Vampire Lord.