Dez, they'll eat anything in the tank the can smack. And they're really fast, so they can smack anything they like!
Have you seen the pop bottle style trap? You cut the top off the bottle and glue it inverted back into the bottle. Shrimp can go in but gets confused coming out. Mantis are really smart though, so if you miss the first time, you won't get a second chance :P
You might get it to come to the surface to feed from a stick and maybe net it. Adam used to twirl his around on the end of a feeding stick!! It would just hang on forever
Good luck though, and if you do catch it, they make interesting "pets" in their own tank