You best bet might be sneak attack from a considerable distance and poisons on your arrows. Poisons to damage health and maybe magicka and magicka regen.

Unfortunately retreating could give the spellcaster time to use a healing spell

You bring up a good point actually I know a mod for Witcher 3 has been released that lets you know the level required before you start it so you don't get lumbered with something you can't complete at your current level. I wonder if something similar exists for Skyrim and if not whether its possible to do the same?

But the running blindly into more danger can be so fun. Cue in Benny Hill music

Isn't that way we all play? (cue https://www.youtube.com/watch?v=ZnHmskwqCCQ)

I can think of many...many thing's that one could not tackle and live through, at a low level... Shear Point?...verrrrrry easy to stumble into, My lvl 3 "pure mage" did...DOA...two second's flat. Forelhost... lvl 10 2h, heavy, archery... 3rd chamber. Sure if you know what is there and can prepare...then yeah maybe with the right gear/ potions/ follower...but 1st time..no death's..

I have played almost constantly from release.. yeah I can get past Shear Point... with the right resist potions, a follower/ summon for distraction at lower levels... but I know what is there. After playing and doing some of the same quest's over and over again, I think it's forgotten exactly how hard they can be, if you don't know what to expect.

Well, in all fairness though, the easiest way for a low level player to get past Krosis is to run like heck. Although there is no warning that he is going to pop out, once he does, his http://www.uesp.net/wiki/File:SR-creature-Krosis.jpg alone should be enough to scare the heck out of any low level character. And although he might be able to one hit kill a really low level character, I've never had that happen. My first character to meet him was fairly low level and yeah, he died, but only after trying to fight Krosis. He easily could have ran away and survived. The issue I see is the overall games level scaling is designed to let a player have a good chance at beating any enemy at low level but then throws a few curveballs like Krosis at the player, which can be surprising if you are not used to running away from fierce enemies.