The hardest part about taming this monster was luring it into our trap. He wasn't interested in us as bait, so here is how we had to do it: get on your argentavis and pick up a tasty helpless creature like a megaloceros or a phoiomia. The giga will chase this creature for far longer than he will chase you or your tames. Have a friend on another argentavis ready to pick up aggro with another wild dino when yours inevitably gets chomped. Oh, and try not to die in the process. Good luck!