How to kill a giga 1, raise 30 lv 350+ rexs 2, find a giga 3, realize that all your rexs got stuck on rocks and spend an hour getting them to the giga 4, kill the giga with your rex army that will dominate anything
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.