How to fix it ?

If I had to fix it, I'll try by adding more unavoidable damage.
- You did the mechanics correctly ? Good, still receive 50% damages to your pretty face.
- Constant DoT to everybody.
- Damage done reflected to players.
- Mini tank-buster to a random player (who maybe will need protection from the tank in hard content)
- More frequent heal-check

Here are the few options I'm thinking right now.

And reducing cost of healing spells so heals don't go OOM too fast. (you know, they'll use them far more often than now)