It's easy your just not fighting the right guy's. you need to be at about level 15+ to get the strong Monsters. you need something like a minotar, use your spell/sword and then CHOP!!! his head off. And that's it.
P.S. if you want to cheat (your choice) then just fill one soul-gem, then use the copy item cheat and you'll have an endless supply.
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