However, if your opponent is so huge that you can't even reach it's face and simply the weight alone can kill you, it gets more complicated.
Then you go for hamstrings & gonads...and if you thought ahead (and aren't planning on eating your foe), you're using poison.
If you REALLY know your target, you may even maximize your odds of success by finding out if a particular toxin will be more effective. Consider my maternal aunt- one bee sting and she's on her way to the hospital.
(And, FWIW, that's a size differential greater than Halflings & Giants.)