The critical hit of the Sin Scissors (SS) is one of the easier critical hits to accomplish. There are two things you have to be aware of.
1) The angle of the scissors. Wait for the SS to get close to you (in range of your "sword"), then attack the SS so that you deflect it's scissors. If the scissors are at too sharp an angle, the SS will recover quickly, if this happens, don't attempt to critical hit it. Instead, back away slightly, and wait for the SS to approach you again. Use the same tactic.
2) Once the SS has been stunned (This is easy to see because the SS will hang with the scissors away from him for a much longer time than when the SS recovers quickly), you have to shoot it in the face. To accomplish this, you have to walk up to it and turn about 90 degrees clockwise to successfully execute the critical hit.