Knocking it out was only slightly difficult, my high level Direbear tanked the damage until I could pump enough tranq darts in to him. The hard part for me as keeping it asleep and alive. I actually built a custom little house next to the unconscious dude and started producing narcotics to feed it. I took the thing down near the base of the bug volcano on the island and it gets freezing cold there at night. I'm fairly positive the extreme cold was killing there creature to I had to sit there and force feed the thing, losing all taming effectiveness but keeping it alive. Suffice it to say the game worked, though it's definitely the hardest I've done, and I still have my Theriz to this day.