Deathsmiles PCB gameplay question

When I fought one of the bosses last night (the head on the ground) his skin blew off and his skull was bouncing and thriving around. The visuals were very cool however when i replayed the level a few times that never happened again. The skin stayed on. What conditions must be met for his skull to be reveled?

Here's your answer:

2) Bosses on Level 3 have an extra phase above and beyond levels 1 and 2 (e.g. Jordan turns into a skull). They also score more points (500,000 for the final phase instead of 100,000 for level 1 and 200,000 for level 2). Intermediate phases are worth 50,000 in all cases on all levels.

Click to expand...

Taken from this post over on the shmups board.

I actually wasn't aware of this. Never noticed the difference as I got into such a groove of playing the levels in one particular order so I could get 1CCs.