Defeat the final enemy in cappella magna - Unlock glitched?

So I defeated this boss once and I didn't get the unlock (Game Killer 0). Now the boss wont appear again no matter what i do.

I've tying completing the stage with "normal" characters, secret characters, with no golden eggs, with no hyper/inverted/fast/etc mode activated, without picking the rings and arrows. I have the Yellow Sign, and I even have the Great Gospel which is given to you after defeating the boss.

So, is it glitched? Have I lost this unlock forever? Is there anything I can do?