I guess a Mystic Mage with Protection in Stone form can be pretty difficult to kill.
However, there are some factors that should be looked at.
First, Cleansing Winds
In order to heal for max, that requires 120/120 Mysticism/Focus(or Imbuing)
If the target is poisoned, the healing done is reduced proportionately to the strength of the poison.
Fastest cast time for this is about 2 seconds without protection.
With protection this should be about 3 seconds.
Stone Form.
Must be on foot.
Casting speed reduced.
No immunity to paralysis.
Protection
Casting speed reduced
Magic resist reduced.
Healing Stone:
Curing poison takes a minimum of 15 Points. (Level 1 Poison)
Max points on a healing stone: 300 (20 Lev 1 Cures)
Now the difficulty is whether or not you can take down this character before they make it to a safe area.