I have had serious Mantis problems. I lost several nice fish. I have caught two so far but I still have a big one in there that makes a lot of noise. Get a clear water bottle 20 oz. or a 2 liter coke bottle. Cut into two pieces about a third of the way from the top or a little less. Remove any plastic ring around the top. Flip the upper half around and shove it into the cut opening. It should look like a funnel pushed inside the bottle. You need a tight fit. Separate the pieces punch some flow holes about midway up the lower half. Fill up to the flow holes with tank water, add feeder fish, black mollies are great but I caught one on grass shrimp, a small piece of shrimp or dead grass shrimp also will attract them.
Shove the top back in place . Find a nice place where you have suspicious activity or the known crime scene. Place the bottle into the sand, bury until the opening is even with the sand bed, with not much sand in the trap. Disguise it some if you can. He is not real smart, he can find his way in, but it takes him a long time to find his way out. Usually more than a day. (the mollies can get out to) Be careful, don't handle him, he can deliver a nasty blow. My local pet store owner got few stitches in his thumb.