With the ability to use weapons on certain mounts being added very soon, I'd say the best way to KO a Giga is to mount a raptor or another fast mount with some levels into movement speed and stamina and shoot tranq darts as you run away from the giga chasing you, you should be faster and in theory this should become an easy tame.
I dare you to raise one!! They are savage if you got two high lvl parents (about lvl 140ish) and you get imprinting on it. The thing will become an adult with about 230% melee and basically one chomp any bronto!!! But on the down side you loose your job and your girlfriend
Make 4 metal dino gates and 1 large bear trap. Place three frames and door into an incomplete square and the large bear trap in the I between them. Lure the Giga into the trap and while its trapped place 4th down to make the square. Then just light it up with tranqs