You can punch them at close range to stagger them, but it's still not enough time to draw and arrow and shoot them. At least not without quick draw.
I went the archer route as well, for long range sneak attacks. However, I'm also proficient in one-handed for when the battle gets up close.
You could also try hiring a follower to go with you for that first Overlord. There's a Dunmer in The Drunken Huntsmen in Whiterun you can hire for 500 gold.
I stopped using destruction magic pretty early on. It has its uses, like shock and frost damage, to drain the targets magicka and stamina respectively. Draining stamina will prevent those power attacks, at least.
I'd also recommend you start leveling your smithing a bit fairly early on, and make quality weapons and armor, a good bow is especially useful, combined with sneak attacks.
Either way, the game starts out a little difficult, then becomes pretty easy as you progress. Too easy, perhaps, as I had to bump the difficulty up to Master for any semblance of a challenge. (Still not getting one, really)
