it's really not that hard...
undead are overpowered
use your mage to dual cast fireball ice snake firerain w/e
kill an enemy stack
use necro call from the your own necro and voila
let your skelli archers kil the rest and use cursed ghost or ancient vamps to kill the left overs
a better challenge would be with a NEUTRAL unit
so no humans(save for barbs robber pirates)
no elves no orcs no demons no undead
u won't get ANY moral bonus so that should be a HARD CHALLENGE