If you ever find yourself smack dab in the middle of a full fledged cat fight, remember, the cat's most vulnerable area is the butlock.
However, it has also been said that cats are found to be sensitive near their front facing whiskers and behind their centrally located, backwards facing ear.
If in doubt, try grabbing their hind leg, but be careful. This leg can vary depending on which way they are facing.
Good luck. You'll need it.