[Tuna-Fish]

There is a spell called dispel (enchantment 5 iirc) that takes an astral-3 mage to cast.
Research it, and set your astral-3 mage to cast it. When casting it asks how many extra gems you should use, when casting the Wrath of God, the enemy had a similar dialog, and to succeed in the dispel you have to use more extra gems than he did.

You have no astral-3?
There is an item called starshine skullcap, takes construction-6 and an astral-2 mage to produce. when you give it to an astral-2 mage, he becomes astral-3.

You have no astral-2?
Tough luck.