Instructions
Enter the alternate version of Stage 3 (Stage 3') and play through until you fight the three-eyed boss.
Defeat the 3-eyed boss. The Axe subweapon is recommended for this purpose; you have to destroy its eyes, not attack its face. Take the lower path after defeating the boss.
Strike the top part of the wall under the overhang where the door is located, then work your way down the wall until it is completely destroyed.
Pass through into the new area and go down the stairs to rescue Tera, who gives you a charm that allows you to destroy Blood Skeletons and Blood Skeleton Barriers, both of which regenerate unless you have the charm.