There's no way around playing through the entire game on the hardest difficulty setting("Gothic"). Here's some advice, credit goes to N7 Johnny Clash :
-You need the lightning spells to stun single enemies and keep them defenseless when attacking them or to buy time to escape.
-Another invaluable help is investing at least 22 points into the "Discipline" branch of your skill tree, which gives you the ability to attack without stopping.This will take care of almost any single strong enemy since you'll trap them in an unlimited combo.
-Also necessary is the strategic use of your ranged weapon.When you get attacked by more then 2 Strong enemies, draw your (cross-)bow, walk backwards and fire away(also works with groups of animals or undead).
-Walking back without standing still also ensures you that you will not get hit by Melee Attacks, when you fight against mages or archers you can use the Melee combatants as cover from the projectiles(However they don't take damage from friendly fire).
-Also keep in mind that the enemies don't follow you forever and above all they can't climb(you can even loose a boss, get out of combat and then to save in the middle of a boss fight).
-In the dungeons near the end of the Game, just run like hell till you're out of there