I have experienced the same as the OP after lvl20, PS3 version of the game. I feel that it is the effect of several things:

1. After lvl20 the leveling system of enemies jumps one category higher, so the bosses are lvl50 (at least!) and up;
2. The engine simply cheats you. As it was mentioned before, the enemy mages are super accurate, super fast, run backwards as fast as forward around all the corners, while shooting at you; At the same time you struggle with controller, cant aim properly making circles around, when they run.
3. The game is simply bugged, nullifying your resistances.

The point to understand is, we think that the Skyrim must be versatile, believable, interesting RPG game. It is not. It is just yet another game for the same price as all the others. It is somewhat beautiful, but has a lame and cheating fighting mechanism and a linear game play.

I honestly Admire you playing a range character! On PS3, it is virtually impossible, mage or archer, with the bloody struggle with the slow controller. Impossible to target anything fast enough, especially moving.