[Taqwus]

Flaming arrows also help. A front line of high-prot, 0-enc, unbreakable troops (e.g. Living Statues, Mechanical Men, Crushers) can help a lot; most undead will have trouble hurting them, except for the lance attacks from the longdead horse.

Killing commanders will help more than usually; kill all the enemy commanders with undead leadership and the mindless hordes will dissolve.

Spells that scale well (battlefield-bLasters, orb lighting, others) will help.