I'm not very familiar with the editor either, but couldn't the Astral Gate be placed on the list of forbidden spells for a map? After all, the "super ability" doesn't include absolutely all spells available in the game in the spellbooks (for example, summoning Il-Hijra). If I'm wrong, I apologize.
On the other hand, I don't see much potential even for a powerful Chaos Mage when it comes to destroying opponents in large numbers. Unless there's a way to significantly increase their spellcasting skill before a battle.