Some useful spells against undead:
Flame Blade (priest, level 2)
Repulse Undead (priest, level 5)
Bolt of Glory (priest, level 6)
False Dawn (priest, level 6)
Sunray (priest, level 7)
Hold Undead (mage, level 3)
Phantom Blade (mage, level 5)
Control Undead (mage, level 7)
Any fire-related spell will have good effects on Undead (they hate fire and light)
You can also buy the Azuredge axe from Bernard in the Copper Coronet and equip your tank with it. Or just get the Mace of Disruption from the blood pool in Bodhi's lair.