In Oblivion I had to rely on luck to beat the last boss on the main quest, the evil elf guy whose name I forget. I picked Altmer and the Apprentice birth sign, which gave me a 200% weakness to magic, so I was only able to win because he kept summoning instead of casting magic at me, one fireball from him would have killed me.
Also, at the end of Knights of the Old Republic 2 the first time I fought Kreia she just used insanity over and over in between stabbing me to death with a lightsaber. The second time I tried it she just used a rubbishy drain life spell over and over and I killed her no trouble.