Use stone dinosaur gates to trap it next to a bank or cliff. Build a catapult on high ground and hit it in the body with boulders. Be careful not to hit it anywhere else as a couple misses will kill it before it downs.
Tranq Arrows/Darts DO NOT WORK!!! To knock this out you need to use a catapult and shoot in only on it's back. If you shoot it on it's legs it will kill it. UP-VOTE SO EVERYONE KNOWS & TO GET DODODEX TO ADD THIS!!!