Despite having a paddle the size of a boat for a tail the Giga is incapable of moving beyond a babies crawl speed in water. Lure it far enough away from land and this mighty behemoth will give up on life and quietly sink beneath the waves.
WARNING: Takes an absurdly long time to actually drown. Possibly do to being a Sith Lord.
If you NEED to kill a wild one with a mount, try to trap it in a taming cage then use an Allo/Carno/Thylo bleed. Or if you have a juiced up thylo, you can run in a circle and bite its ankles, but this is hard and risky to do.
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.