Spellcasters are some of the toughest and some of the wimpiest critters in the game. More often than not, they try to hit you with an area-effect spell and wind up blowing themselves to smithereens.
Spellswords, Nightblades, Battlemages, Healers: Spells can get you at range and theyre pretty handy in melee. At higher levels theyll spell you first and then close for melee. Battlemages can get off a couple of decent spells before they close. Spellswords and Nightblades like to leech your magica, shock you and paralyze you. Spell Reflection can take care of a lot of this. Except for the Battlemages, missiles are a good defense. Youre probably a better archer than they are. Treasure usually consists of a couple of armor pieces, a weapon or two, money and potion ingredients.
Sorcerers: Sorcerers are pussycats as long as you dont throw spells at them. Since they cant regenerate magica, they have to absorb it. As long as they dont have any theyll engage in missile fire or melee and they dont have many hit points. Bethesda could have made these guys tougher by giving them some magica to start with.
Mages: Surprisingly enough, Mages like to get off a spell or two and close for melee. Maybe they think theyre Warriors. Anyway, they cant stand toe-to-toe with anyone for very long, but their spells can put a severe hurt on you. Treasure is usually money, a weapon, and lots of potion ingredients.