This is story-related and can't be missed.
This will unlock the moment you destroy the boss of Issue 4. The boss is in a huge, gold-coloured room, and as you do damage to him, more and more of the annoying, seemingly-invincible purple movement orbs come into the room. The boss is invincible normally, but when he charges, dodge him, let him hit a wall, and then he'll be dazed. This is the time to attack. Use X-bombs when he hits the wall and it'll take off about 20-25% of his health each time.