It depends on:
1. What kind of flashlight you use
2. Where you hit them
3. How you hit them
Basically:
1. Have a heavy and hard light (Mag-light, etc.)
2. Hit at major muscle mass (inner or outer thigh, top of forearm) or, hit on joints (elbow, knee, wrist) or, surface bones (back of hand, shin, etc.)
3. Hit hard and stick to the target when hitting muscle -- you will have sufficient penetration to incapacitate your target. You can snap at joints and bones.
... And with sufficient training and motivation, you should be successful.
ML