Move over and up to the old temple. Then get ready for a very big fight.
Rhulana and Forgrimm should be able to handle a zombie quickly by themselves. Anything you and Dranor/Gladys can do is icing on the cake. I wish I could give more advice, but this will either be easy or hard depending on how well you're set up for combat. Fight your way past the first few zombies and then set up by the front. These 3 crypts will continue to spawn zombies at a fairly regular rate. You'll get experience for each kill, so there's no rush to move on until you're sick of fighting or you're getting too injured.

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The only trick is keeping your party together and tag teaming the zombies. You need to thin them out quickly so you just have to take a few at a time. Once the front of the temple is cleared out enough, you should walk through the front door to find the necromancer and trigger the cutscene.
Just stay out front for a minute. Do not blindly charge into the temple and attack. Attack the 3 skeleton archers who should run out to fight you. Keep them at close range to neutralize them. Kill them while holding off the zombies with one of your warriors. Then get ready for the big push. Rush forward down the right side and just get to the necromancer. Then have the entire party attack him. Focus on him and have yourself or Forgrimm use knockdown attacks to prevent him from using his magic. Once you kill him, all of the zombies will die and the spell will be broken.
This officially ends the zombie plague itself, so that should open up a few more quests. Report back in to Rakorium to let him know that he's in peace for the time being. He'll start up a new quest though. You'll need to handle “A Magician's Worries” before you can officially advance this one.