making bosses better, how could it be done?

I think a boss that shuffles what kind of attacks you need to damage it would be kind of cool.
Like... in one mode, only warriors can hurt it, in another, only mages... It would promote a solid party because you couldn't beat it if it changed mode and you couldn't damage it....

O.o We already have that, yes its only divided into physical and magic damage but it's pretty much the same thing.