Kill Margit with 6+ parries
To complete this square, you must defeat Margit, the Fell Omen, after performing six or more parries on him. You do not need to take the ripostes that are available after each two successful parries. Stance breaks that do not come from parries do not count.
Parrying Margit is a skill that varies depending on your equipment. Small shields with parry are usually the best option, but if they are not available you can use the Storm Wall Ash of War (dropped by a scarab on the cliff between Stormveil Castle and the passage to Liurnia) or use a medium shield with parry (the hardest option as there are fewer active frames for parrying).
Margit counts as a Limgrave/Weeping Peninsula boss.