To immobilize/stop it from moving,using a bola is your best bet. For knocking out,I suggest using tranq arrows with a basic bow. There is less chance of it dieing that way. I'm a higher level in game,so I built a tame area with behemoth gates for dropping in wild tames with a quetz. This makes it easier to knockout and tame without the added worry of attacks.
Taming these guys is a pain, since they stay asleep for 10 secs. But the idea to keep these guys asleep is by bringing bio toxin. Since it has a lot of torpor when consumed,. But what sucks about biotoxin is that it spoils. Up if this was useful.