Using the cannon is better. Make a trap and get a markings in there to see where and how you can manipulate its direction,once u figured it out put the cannon at a safe place.build the trap a little higher makes it easier to aim.90-135 are a single cannon shot.
Pretty easy to tame. Trap it in a circular form of stone Dino gates, place catapult low if catapult is high the crab will die. Then shoot it with a catapult. Cannons insta kill the crab, do not use cannons
I understand that all the previous tips submitted were useful, but just in case it was not clear, hit the TOP OF THE CRAB, it will have a special animation if you hit it in the correct spot, and will take only a small amount of boulders to tame, even a high level. (Irrelevant with Cannons)