GI ANTS. Government Issued Ants. So yes. Unless you have bug spray. Stay the freak away from GI ANTS.
I killed my first giant as soon as I stepped outside White Run. They're easy.. Just fall back or climb high.
All you have to do is spam them with arrows. You can run faster than they can, or like neonwolf said, just get high.
I just shot my bow, and used unrelenting force. Once I did tat, my first giant got stuck between a rock. Easy to kill then.