Haha, let me know how it works!
If you're up against ultra-greatswords, it's cake to parry! Those are so slow you can actually parry when you see them initiate their attack, rather than predict it. And it's extremely satisfying to pull off, since you actually timed it rather than random-parry. If your opponent is wielding a huge weapon, be ready to parry!
If you're up against smaller weapons, it's best to try and parry when they start spamming R1. Otherwise, since their attack animation is so quick, in most fights you'll have to predict the parry, and that isn't so fun or rewarding. I've had a few fights where the lag was optimal and I was able to parry by looking at my opponent's swing, but those are rare. When you're up against a small weapon, don't give your intention away. Let them spam R1 before trying to parry. Otherwise, if they know you're out to parry them, they might be vigilant about the spamming, and good luck parrying then!