In the beginning of the game get a bellsprout and turn it into a weepingbell (turns into one at level 21). then when its the day for the bug catching contest use weepingbell as your one pokemon. Weepingbell should have cut, vine whip, and sleeping powder. Weaken it to about 10-1 hit points left ( estimate the points ) and use sleep powder. throw a park ball and you have a very high chance of catching a scyther/Pinsir. I caught a scyther on my first ball! I wound'nt use stun spore just because you would have a greater chance with sleep powder.

Note: this technique works on the other pokemon too.