Regroup with Percival, as it were, back in Medieval England.
Follow him to lay the smack down on a group of knights.
Rinse and repeat as you head toward Nueva York.
This ends with a showdown again Black Knight and Enchantress (again).
The two will keep swapping back and forth. Against Enchantress, destroy her clones, then attack her; against Black Knight, strike after he's just finished a charge, and is vulnerable.
They both have a lot of health, so you'll back bounce and forth between them several times before the fight ends. When it does, you'll unlock Percy for purchase.