Due to his Master of Necromancy spell, Zoldark usually has a high armor and maximum health bonus, so a direct assault is not recommended. He does however lose health whenever he resurrects one of his minions, so it's best to concentrate on them.
His health does not regenerate over time and there is a resurrection shrine right before his chamber, so if all else fails the party can simply rush in, hopefully kill a minion, wipe, and then repeat. Another way is pretty much the same, but continuously fight 1 minion, kill it, wait for it to resurrect, and kill it again until he stops making minions.
Since all of his monster skills are spells, Backfire will always trigger.
Holy damage is recommended, since it is doubled against undead foes.