Hello Sigma88
I`m glad you`re having fun still. About your suggestion for save game, post it here and i`ll try. Just post what version are you playing, and if you`re using any mod (i was planing to play again soon, and will try to help).
As for advice still low intellect for mage, his best weapon isnt army, but spells with units with controlling abilities.
Karador have all undead army, and they mainly have great poison resistances, and some physical, only necromancers magic. So it good idea to use units with high magic dmg per leadership (inquisitor is great here, for dmg and holy attack and resurrecting your fallen troops, druid or dryad also magic dmg and summon units to harass enemy). If you pick some of them, lose demons and get some other meat shields knight or horseman are good choices. Also giants arent bad ides since only ghosts are flying and his jump attack will dmg all undead units, run as close to enemy you can and leave him there. Cast magic shield (archmages) and peacefulness spell and rage skill gloth armor no him and just jump when ever possible.
Post here save game, with version/patch info and i`ll try to help you. Take care
Edit : or sen save game file to
ladovinka@gmail.com (my mail)