Getting that weapon is story related.
as soon as you get it, put it into your equipment slot.
equip the weapon now and look for enemies.
how to parry:
when an enemy is attacking you, his weapon will flash white. right at that moment press
ofc this cheevo is glitched af. i got 70% by just evading enemy attacks and the last 30% doing 1000 parry with multiple enemies.
if your counter stuck just search for another enemy.