Personally playing a Mage-thief type of build I like to call "The Puppetmaster"
My three main perk trees are Conjuration, Illusion and Archery.
Using Breton as a race for the magical resist as thats pretty much the only thing that hits me.
I basically play it as I would an assassin/thief character, but the mage version:
Invisibility + Muffle is your stealth.
Bound Bow is your main source of damage.
Early game:Your summons are your damage, your bow dont do much and you dont have invis nor good stealthing, so you are extremely fragile. Just keep a summon out at all time and use a bound sword or something. This is true untill you get Bound Bow.
Mid levels:Sneak in, get behind ennemies, summon your bound bow (silent cast perk), sneak attack then re-invis. The "Calm" spell helps a ton here. Play it 100% assassin with your summons being there -only- if the [censored] hits the fan and you cant invis away, or against hard ennemies that require more then 5 arrows.
Late game: PANDEMONIUM! Invis in, AoE Frenzy, Summon 2 Atronachs, Invis out. Of course you still have the option of simply sitting back in the shadows and lobing arrows into the pile
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Overall an amazingly fun build.
Also: With that build I use the Slow Time shout almost exclusively.