I don't have a guarenteed method, but copied this from another recent thread. Magic_carp replied this: "lol, i know what you mean, its almost impossible to catch sallylight foots, in mexico they are all over the place, i used to catch them for fun when we go on vacation and this would work, they come out of the water and chill on the rocks and eat algae, get a stick, and you can usaly find old fishing line on the beach, but tie the line on the stick, have long line so you can get it close to the crab, i used fresh snail meat but you could use shrimp, clam, etc, tie the bait onto the other eand of line, tie it tight so it doesnt come off, and literly fish for the crab, it will grab it and wont let go, pull it out of the water fast but be carefull not to forcefull, put the crab in bucket with tank water, and try to push it off the bait, or just cut the line where the bait is so it can eat it and it wont be to stressed. I did this to get several Gorilla Crabs out of my tank
hey it not only works, but you'll have a blast doing it".
Maybe that will work, who knows, but worth a shot.