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.
@Stormwebb19

Cannon balls discrease taming effeciency significantly! So for high tame level, always use boulders. Please let other people know! :)

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

Your crab is out of HP, and there is no annimal corpses to bring? Just put 10+ sleeping bags not far from the trap and let Karkinos kill you enough times to regain HP (as I calculated - 300 HP per death)

This works only when Kark is not fleeing. Otherwise you need to wait until torpor goes down and crab starts to attack. But this worth it. You won't lose the crab you want!

To the guy who said "rare on floaty islands on crystal" that is a load of crap found 20 on 1 ISLAND!!!

I just figured out that rocket launchers deal torpor to the karkinos, much like the rock golems, but they die a bit more quicker since they don't have much health and rocket launchers deal a lot of damage.

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)

This tame is like the Rock elemental, but do not shoot it in the head. Shoot it right in the back of it's shell. Anywhere else will kill it fast.

If you are missing with your catapult and it's dying, drag dino corpses to it and let the karkinos eat the corpses. It will heal him up a lot then try again with the same torpor you left off with.

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.