Thuzadin Necromancers are powerful as they have a few potent spells at their disposal.
Spells they cast:
Summon Skeleton
Drain Life, drains 110-111 life from the target and transfers it to the caster, lasts 15 sec.
Bone Armour, makes the caster invulnerable for 15 secs to magic and melee damage.
Strategy
If you are afflicted by the Darin Life spell, just move out of range, the spell has a range of 20 yards. When they cast Bone Armour, if there is a Warlock, use his Felhunter's Devour Magic ability, or use a priests Dispel Magic. Other than that, they go down easily.