Don't fret too much about the torpor decay. It's actually not that bad even though it's scary when you look at it. While the decay rate is incredibly fast. The recovery rate of narc is extremely slow comparatively. So while you'll have to rush it to drop you tranqs, wait until the torpor is quite low (I've been told 100 but I don't trust my lag as much so maybe 1000 before pumping it back up) you'll then have the easiest time in the world to tame it as you'll easily have 2 hours before you need to come back and put him to sleep again.
Make no mistake the difficult part is actually putting a ko on the creature. If you got two player the cheapest way that'll work is to have a person on flying mount hoist his buddy with a grappling hook.
Bring spare weapons with misses and the crazy torpor decay, the estimated number of dart to take him out is wishful thinking at best.
Solo, make 4 metal Dino gate + bear traps.
Bring sleeping bag / bed you'll need them.
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