[Twan]

A line of ae buff spells making troops illusion immune or low level big ae spells doing 1 dammage would be sufficient to balance the game, if well distributed between paths.

Some ideas :
low level evocation fire or water spells doing only -1 AN dammage (max 1 hp like a whip) but in a large ae
low level nature alteration spell "owl eyes", medium ae, giving darkvision + making troops illusion immune
low level astral thaumaturgy or alteration spell "true seing", small ae making troops illusion immune
a level 5 version of true seing, large ae cost a gem, making troops illusion immune
a level 7 version of true seing, making all troops on the battlefield illusion immune
high level ritual version of owl eyes or true seing, make all troops illusion immune on a province for 1 + (extra gems) turns and give a patrol bonus to these troops
(illusion immune troops will also have bonuses against phantasmal creatures and some spells)
high level earth BF enchant "Truth of Weight", the earth only allow creatures having a weight to continue to exist, dispel all mirror images as well as phantasmal warriors and false horrors