You can always use Wards spells from the Restoration school to protect yourself against spells and dragons breaths, unless your character is pure melee and refuses to use magic of any kind.
You're carrying a shield, use it. Shield Bash your opponent to interrupt his or her casting. Since you have Unrelenting Force, use it in conjunction with your bashes.
Personally I'm not pro-potions, but if you have some resist/regen ones in your inventory, use them.
Eat your own ego and hire a mercenary for the fight. You'll be surprised how easy things are with one around. The Drunken Huntsman in Whiterun has one for hire (500 gold pieces).
You can always use other enemies to block the caster's line of sight. So if you're fighting against a group, hide behind them and finish them off before going for the caster.
Use your character's racial ability. They can be life savers when used properly.
Leave the dungeon and come back a couple of levels later. That will make things easier as well.
If you're still having troubles with that, turn the difficulty down a bit, and try again.
I find it rather amazing you feel like rage quitting the game and blame the company because of your own incapacity. There are several ways to get rid of an enemy, you're just not thinking outside of the box. Pushing enemies into traps; using oil lamps to ambush them; use corridors to fight only one enemy; you name it.