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 up/vo/te so people can see
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)
Put a catapult on a paracer, bring a high health turtle (carbomemys), whistle attack with the turtle and when the crab hits it put it in passive so you can easily aim and hit him on the body. No weird engineering needed.
People were commenting boulders are better than cannons for taming these so I just assumed they meant rock golems. Kind of went out of my way to tame one. Doesn't seem to work for this but eh, I got a golem now. Lol
Make sure you know where your Larkin is is before you set up your trap. Do not lead them to your trap on foot. Kill EVERYTHING in the area before leading them over. Place down bed or sleeping bag. These tips will save you a lot of stress