Having a crab already makes getting more crabs easy. Make a trap, lure a crab into the trap with a parasaur. (Aggroing it on you will work, but it's not 100%, better to grab a parasaur or something like that) then get the wild crab stuck in one section of the trap so it can't move. (Body block with your own crab, it most likely won't even attack your crab)
Bam. Easy lure, easy body shots.
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!!!