Perhaps a bit late of a reply, but anyway :
I recently started all over with a mage build (with some sneaky stuff thrown in the mix that I plan), and had the exact same issue as you had.
I solved them through several ways :
* I stopped spamming destruction spells like crazy, and always keep some magicka pool available
* If I have a follower, I run back to her and circle around her so that she takes over the opponent with weapons while I rest
* And the most usefull thing of all : the Calm spell. I now always keep enough magicka for a last Calm spell, and I spent the illusion perks necessary to have these spells efficient on higher lvl NPC, and still cheap in magicka. It is an incredibly helpfull spell at low level, you have all the time needed to regen your health and magicka while the guy is quietly going back to his original place.
* And before any crowd engagement, I "clear the crowd" by sneaking rather close, and cast Fury on the farthest ennemy. Farthest because by attacking the closer creature to him, it'll never be me, and often, the farther is the boss, who, being tougher, kill all others. While he does, I sneak back so that he doesn't come after me too fast. If 1 catches me Furying his friends, I Calm him, and hide.
Illusion is a great survival school
Same issue here, on master difficulty, and I too found Illusion to be invaluable early on. It's not the solution to every problem though, like in dungeons where it's practically useless until much later in the game, but otherwise it helps a lot. Fury is especially fun, but it works only up to level 6, however Calm is amazing. Before, I would get ambushed by a big cat, witness my merc die, and then be forced to run away with no magica left from trying to kill it and of course die soon after, but now a simple calm will do and I can move on. I think only 4 perks are needed for Calm to work most of the time too, which is nice. It almost feels like cheating sometimes, because no one will cast calm on me.