Fun and Challenging Bosses, How would you do this in a Turn Based System? Options

[Shaddow]

So, I am having trouble, as you would have guessed. I am making a turn based battle system, I know you think that's boring, I disagree. Well, except when it comes to the boss fights. I can't see to make them interesting and challenging at the same time. My question is this: What would/do you do for a turn base boss fight to make it interesting?

I read up on the boss type tutorial and such, it was helpful in knowing the different types, but with the limitations of the RM system, how do you do that? What can be done to make these interesting?