As far as I know, the "critical" is only valid for the attack spells.
I have a low level EP char (lvl 29+), with which I have already healed, buffed and revived many players (and sometimes even getting paid for it, although I never ask for any payment).
I have experienced my fair share of critical hits with my attack spells, but have never seen an...... errr... critical heal.
Kind off a weird choice of words --> critical heal
Would be awesome though if it were possible.
~Delphi~
Lvl 75 EP