help me with Impossible mage!

[ihira]

My first playthrough was on Hard Warrior and while up until the Big spider was pretty challenging, as soon as I got Warrior Maidens + Veteran Orcs almost all battles became a 2 turn no loss battle.

So I'm trying Mage on Impossible now but holy *** how do I beat the necromancer in cave (after Ahriman).
Right off the bat he does a ghost blade that crits no matter what.
My leadership is only 554 and can't produce much physical damage.
This is against Ancient Vampires that regenerate, and Ghosts that cut 50% physical dmg.
The worst is when my army is near death he will use necrocall to ressurect a fresh new troop that takes me turns to kill. wtf

My current approach is lvl2 natures call to get 945leadership Griffens to tank (+counter) and then use lvl2 healing until I run out of mana...but yeah he casts necrocall its over.

I have the best equipment available and the spell selection sucks too.
ANy suggestions?