I love this concept!
I was always a healer in MMOs and I absolutely loved it.
This game feels a little too fast paced compared to what my nostalgia remembers. It was all about manna efficiency, and keeping up the HOTs, with the occasional shields and a well timed big burst of healing based on how well you knew the boss's attack pattern.
I was too busy spam healing and button mashing to really notice the boss's attack patterns.
Anyway, I will check back on this in a few months to see if you change any balance or add more to it.