I would recommend phases like how i did to the final boss of my upcoming project..
It could take some time... and certainly a huge-load of testing.
You just need to use events and layers
So here, you just want like a cutscene so your character can undergo into the next phase, and you want to have them take multiple hits. So here, you use the layers for the phase bosses will appear (so they wont appear in the first phase.)
This is how its going to go, The layer you used for the second phase should be used for the event for the 2nd phase, so for the first phase boss, you just put the 2nd phase event into the Death section of the Events tab for your boss, so it would look like your boss is undergoing the second phase.
And there you have it!
If this long speech doesnt make sense, here are some pictures of examples:
Download the examples below to see them.
http://www.mediafire.com/download/c35vy ... ample1.jpg
http://www.mediafire.com/download/autl3 ... ample2.jpg
http://www.mediafire.com/download/7jla4 ... ample3.jpg
btw, speeches/tutorials are pretty much hard to say when it comes to these types, but small ones that i can handle.