You just have to use the terrain to your advantage. Mammoths are easy if you can keep to ridges that they struggle to climb, or get up in a fort. Fort Greymoor has some great platforms that the mammoths can't climb, and when they get close enough you can even hit them with a sword. Though they can gore you back at that range, so step away after.
I go with fire.
With giants I just lead them to the gates of White-run so the guards can help.
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