Advanced Military: Magic

The most obvious offensive magic one can use against an enemy are Blights which feature a continuos penalty to resource production, and single-use spells which destroy a number of resources. When cast in combination with your allies, and applied repeatedly in the case of the single-use spells, one can seriously cripple a city's stocks and resource replenishment, wasting their gold and possibly time queuing troop builds, and ideally forcing them to downgrade sov or troop support buildings as they cannot support the hourly resource costs.

Knowing the troops and items being produced at a certain city allows you to choose the most damaging blights; it might be worth casting Rapid Oxidization (-6% iron prod) and Tenaril's Pungent Solvent (destroys iron and swords) on an infantry city, for example, or Sharp Frost (-6% food) and repeat castings of Plague of Locusts (destroys horses, cows, and food) on a high-pop city whose food supply may be very easily disrupted.

The most popular choice, especially useful against high sov military towns, is usually a combination of Carrigen's Infestation and Carrigen's Mental Lethargy, which will together destroy and reduce the flow of research points (reducing sov capabilities) mana (potentially draining your enemy's geo/blight shields) and books (which are heavily relied upon during wartime to feed high sovereignty's research point requirements, and are also needed for spells and diplos)

Since sov is key in war, and since it's useful even against non-combative cities who may have high production sovs to feed military accounts, this is a combination that is guaranteed to disrupt any sov-claiming town regardless of size or your knowledge of its workings.

However, geomancy can be used offensively too. One can override an enemy's geomancy bonus by casting a geo spell on their city, substituting a bonus your enemy is dependent on (like Nature's Bounty's +8% food bonus) for a relatively useless one (like +3% wood production)

To guard yourself against offensive blights and geomancy spells, military players usually cast blights and geomancy spells on themselves using as much extra mana as they can. Your enemy will have to spend more mana than you used to overcome your spell, whether they are casting a single-use blight or sustained geo spell.

Running a -3% resource blight to a relatively useless basic resource in your town is far preferable to risking the repeated destruction of valuable items like books, swords, livestock, and the rest.

Just remember that the distance restrictions of magic mean these 'blight/geo shields' are only necessary if your towns are within casting distance of your enemies.