Okay I discovered this on a lone Giant I found roaming without Mammoths or anything. As a result he was NOT aggressive. I cast Ice Form shout (level 1 since that's all I currently have found) and then after hitting the Giant he still didn't aggro me so I could just take him down. I know where's the Glitch prefix asshole, well read on. Not knowing an easy way to test if this was a one time fluke or not I went to the Giants hill, Guldun Rock. There were two giants though so if I did one the other aggro'd me. Unlike with the NonAggro'd Giant I learned that if you Ice Form an aggressive one they not only freeze but also fall down in their place. You can easily take the Giant out as it stands back up.