If you are to be faced against a giga, run or fly away. If you have one, put down a metal bear traps(s) while your running to keep it trapped until your out of its aggro range. Don't ever look back. If you choose to fight it, be sure you have two 230+ Rex's fighting it, or a tamed giga 50 levels higher then the wild to guarentee your tame will not die. Taming is a different story, I've only had 13 successful tames, all above 2 hours long and well over 1000 kibble each. My tip is get it trapped in a pen, with a metal bear trap then just spray it with tranqs until its KO. Have extra Kibble and Narcotics on standby just in case, and if it's a 90+ wild, don't try it unless you have over 5 people .
If one is encountered near water, you can simply lure it out to sea with a flyer and it will swiftly drown. Given the water is deep enough. If encountered inland not near water. Pray to the overseers you have something fast enough to out run it. Then promptly die because they're as fast as a mad Hispanic mother with a shoe. Not as deadly as one though.