Like Madonna said, use two handed R2 attacks to bring down the knights quickly. Especially when you see the Knight raising his sword straight up.

Another tactic for the Stone Knights without fetching magic of your own: NPC Summon Spoilerif you're in human form you can also run through the part of the forest leading to the Moonlight Butterfly. Once you enter the building in the back (the one leading up to the boss) you'll trigger a Stone Knight to wake up, but you can ignore him if you're quick. Enter the building and immediately whack at the bushes underneath the staircase. This will reveal a summon sign for Witch Beatrice. In the meantime, hold off the Knight by blocking or running op the stairs. Once summoned, Witch Beatrice will quickly take out the Knight. Afterwards you can take her through the forest as a helping hand.