Just as stealth needs sneak and warriors need Armor, Mages need conjuration. Summon something large like an Atronach in a hallway, use heal other to keep it alive, shield spells for him, and use ranged attacks. If you need to block summon a shield + dagger, then dispell it after. Thats how I played in oblivion. Sometimes I would use a summoned weapon to apply a poison. Magic is versatile, its generally not very effective to just spam destruction spells.
Exactly. I used Daedroths and Storm Atronachs to distract the enemy while I shot a variety of destruction/restoration spells from the background. It worked in every single situation except for Mankar Camoran. I was roleplaying with a Daedroth at the time, and he couldn't keep up against the respawning son and daughter. Mankar himself would often reflect my spells, and my character had high level destruction spells, but little health... a bad combo. I also roleplayed by not carrying more then a few potions at any time, and never carrying a weapon or armor. I remember that battle lasting like fifteen minutes. Raven and Ruma probably died twenty times each before I was lucky enough to get a couple spells on Mankar to take him down.