Getting out of stunlock:
Greatswords (especially the Moonlight Greatsword) are notorious for their ability to stunlock an opponent to death. However, there is a very simple trick for getting out of stunlock. When you switch out your left or right weapon, as I understand it, your poise meter resets, giving you a window of opportunity to roll away out of stunlock. However, it might be noted that, even with this trick, you're all but guaranteed to catch at least two hits.
Preventing chain backstabs:
Often when an opponent successfully executes a backstab, he/she will approach your character as you get up. Many times they seem to execute a second (third, etc.) backstab at this moment. As I've seen in a few Youtube videos (which I can't seem to recall at this moment), the only sure way to avoid this chain backstab (which is usually lethal) is to take simply stand there and take a second hit. There is a mechanism in the game that prevents your opponent from successfully executing another backstab the moment you get up, as long as you do not touch any button on the controller (ex. roll away, lock-on, move, etc.). While it may seem counter-intuitive, this is the only method (to my knowledge) that can prevent the chain backstab.
I'd really appreciate any comments or critiques if I'm incorrect in anything that I've written. If you have another tip or trick for PvP, feel free to add to the discussion! Thanks!