I discovered this method to kill Giants fairly easily, even if you're a low level character.
How to kill Skyrim Giants - The approach is to lure Giants to an area where they can't follow you but where you can bomb them with arrows or magic. Finding such a place is easier said than done, but here's an example that's simple to replicate:
Near Whiterun, you'll find some areas nears the Giants that have small bridges crossing creeks. Lure Giants to these areas and position yourself under the smaller arch of the bridge. The Giants can't get you, but will hang around all upset. From your safe nook under the bridge arch, you can pelt the Giants with arrows or destruction magic at your leisure. You can even hack at them from this position relatively safely. When their hit points get low enough, they may run away, and you may actually get to chase the giant to finish him off.
This approach works just as well with Mammoths.
Here's a video of someone using the approach:
(:youtube Jl8Bzdo3DKc:)
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