Any boss that requires some form of movement can be tricky.
Though you should prepare to actually dispel stuff; not just with your priest but with your 4 mages on Lucifron and Shazzrah - that's what I do when I'm going raiding.

And Ragnaros... just place 2 mages each at a wall and the priest somewhere close... no need to actually flying around.
There are enough spots where you can stand at a wall and still cast.