Right now the only magic using mobs are either the Elementes you find at certain obelisks and wizards. Although especially wizards can be quite difficult to deal with if the have some good spells to use they still seem rather unskilled. The walk up to you in a certain range and than start shooting any magic that randomly comes up with little to no regard as to the kind of magic that it is. The only spells they use with regard to the type properly are healing spells. Often this works if they are just using projectile spells but they are useless at keeping their distance. I would like this mod to feature worthy adverseries that are capable of using the right magic in the right situations.
I have two separate suggestions for this: either elementals or guardians of one type, but those would be limited by the specialization of the magic types, or even better magic grandmasters or warlocks of some type. These would be found in a certain structure arena like to allow for good fights. First of all these bosses should be able to move dodge and walk away from you to make closing in with melee hard to nearly impossible. I suggest splitting magic into different types: spells against groups of enemies aoe, long ranged smaller spells that can be cast quickly, more powerful long ranged spells, close spells, spells that defend or disabilitate the enemy, buffing spells, additional movement using spells and summoning spells. All bosses receive at least one random spell for each category. Then they can use the long ranged spells at a distance the defending spells f. E. To use bubble on an enemy to make a getaway using Teleport and while the enemy is disabled summon minions or cast a powerful spell.
The random spells would make each fight unique and challenging.
As a reward I suggest a new enate amulet or ring. You can only wear a certain number of these. One spell can be passed onto them like a scroll. Casting is via a keybind and their cool down is a bit longer.

The spell type system (attack, defence, projectile, etc.) was originally added with the intention of making wizards more intelligent in future. An AI upgrade for them is still planned, there's some work to be done for that but once it's working for wizards it would be simple to apply to other mobs.