Build a cage (made by pillars and ceilings) near their spawnlocations with a catapult beside it. Then get him in to the cage and start to shoot bolders on his mainbody (If you hit his legs will it do massive damage and no torpor...and do not try to shoot it when it moves, its really risky)
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!!!