winkio handles Blizz-ABS now, so asking requests for Blizzard is, as you probably have guessed, not worth it... Well, let's see...
I don't really think you need a script or anything to launch a boss. I mean, all you need to do is make the boss yet another enemy in the Database, and when you want it to be to appear, just rename the event via a Call Script with \e[ID# OF BOSS] in it. The manual has a section with script references, and if you look at 5.5.4, there's a call script function where you can rename the event. o.o
$game_map.rename_event(EVENT_ID, 'NEW_NAME')
Now, I'm unsure about the multiple HP Bar things, that's probably more suited for an add-on (And it sounds a lot like Kingdom Hearts anyway
).
But yeah, you can launch a boss that way. The manual has a lot of wonderful things inside it. xD