Until you can get high armor and block ratings and can take them on with a melee weapon when they close. I had already slain dragons and though I was bad a** at about 12th level. Even with archery and not using unclimbable (For them) vantage points my arrows didn't do jack. I got KaPowed so many times I got skittish when I got my first giant bounty then found I COULD take them on. And the first time I took on TWO! Whoo I was nervous.

Raise an Oblivion Gate and use your own personal army of unholy warriors to send its soul to the black waters of Oblivion along with its mammoths and the rest of its brethren giants.