Here are some good ways to kill frost trolls:

Combat
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 Use a shield to block the frost troll's attacks. You can also stagger them by pressing attack while holding block.

Weakness
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 Since frost trolls rely to the cold, use flames on the frost trolls. You could use a flame sword, a flame spell, the fire breath shout, or summon a flame atronatch to kill it. It's that simple.

Extra Help
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If you still can't fight, get some followers! Use some of the tools above to help kill the frost troll. Make sure your follower has a weapon with the flame affect or if he or she's a mage cast flame spells, or even a flame staff! 

Extra Feature
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There is a book that tells you how to kill frost trolls and how to protect yourself against them. It's called **How to Kill a Frost Troll**. It'll give you more information on how to defend yourself and fight frost trolls.