Arcane Nexus Tips
By the time they cast arcane nexus you’re in a bad spot. Best to try and pre-empt it. Maybe mass raiding to deny access to gems? If you find any provinces with 2+ astral gem per turn then you can hit those over and over.
Also the caster is an astral mage so you can try soul slay or magic duel. I once saw an AAR where someone teleported in 20 chickens (astral 1, but they have 2 forms so they can die twice) spamming magic duel and took down an astral 9 supercombatant pretender.
To sum up, Arcane Nexus is a spell that considerably increases income from astral pearls.
I have a solution. Cast Disenchantment (enchantment 7), a mass dispel spell. Well, in my current game it’s probably too late, I’ve got more or less 20k troops nibbling away at my territories. But I’ll keep this option in mind for future games.
There is also “Arcane Analysis” in Thaumaturgy 9. That would give you the info of how many gems you need for a dispel or disenchantment.
Though the chances are high that the enemy is just going to put up Arcane Nexus again. That spell quickly repays it casting cost.
Arcane Decree, Alteration 9, would let you put up a global that would weaken any new hostile globals to prevent a strong Arcane Nexus from being recast.