There are actually several ways to catch a mantis.The easiest and most successful method is the use of high explosives.But that tends to leave things somewhat messy.
Coke bottle traps are supposed to be effective.But you still have to get a net over the trap before the mantis escapes,and they are a hell of a lot faster than they look.
My last method proved to be fairly easy.I got the mantis used to eating out of my net,even though I wasnt fast enough to get him out that way,he would just sit on the rock and watch the net untill I was out of sight.I tied some fishing line to the net,placed food inside and sit back and waited.When the mantis entered the net to feed,I quickly pulled the net out of the tank using the fishing pole.