I spent about 56 tranq darts and around 100 tranq arrows to knock out a level 140. Mainly because a lot of the darts and arrows were not registered as hits. I found the best place to aim at was the head I wouldn't waste my darts trying to hit the tail from above as they rarely registered. I had it trapped in a pit style trap so I was above it shooting down.
Tranq darts are ideal, though tranq arrows work fine as well as long as the creature is at full health. It's relatively slow, so you can score some easy shots on foot, though it's adviseable that you aim for the head and nowhere else. They'll take less torpor when shot anywhere else.
Also, bring a good amount of narcotics or biotoxins to keep their torpor up until it's all tamed up.