Unfortunately I haven't seen much development in this area - ocasionally the AI announces they've cursed one of my cities but curse city is a hideous waste of mana - I've never seen spells like freeze, tornado etc being used.
I do remember Curgen's volcano being used against me but that was in WOM - I can't even find the spell in FE.
The AI DOES use strategic buff enchantments on units, champs and cities though so there has been some development in strategic spells - however I have seen cities that have been unenchanted possibly due to a lack of mana
I worry that the AI has been programmed to repeatedly cast spells like celerity thus completely shooting itself in the foot but I'm not sure - maybe one of these games I'll closely monitor AI mana use in a high mana game.
edit - I havent seen the AI use terraforming spells but they could always do this out of visibility
Overall I feel this is a weak area of the AI