1. Taming. I suggest getting a flying mount with good stamina (argy) and using metal buildings to trap it. Then just shoot it
2. Tamed. When the Giga is tamed it loses around 65k hp. And loses a lot of attack. The giga has a bad turn radius thanks to its massive size but makes up for it in brute strength and power.
3. Rage. When a tamed giga loses a lot of hp in little time it will turn into a rage mode which is noticeable by glowing eyes. When in rage mode it will kick you off the saddle if your on it and kill you and your tames.