I recommend the Devs change this spell to have a typical casting time (same as a Heal spell, as it has in pen&paper), but for balance reasons give it a 60 second cooldown.
This helps make it a useful spell to fire off in combat, but prevents it from becoming an 'easy-button'. In a quest like Enter the Kobold or Shroud 4-5, where the party is taking large amounts of damage, using this spell wisely the one time you get to cast it in the combat may make the difference between success and a wipe.
It also brings back a little of the feel of high-level pen&paper casters - where the highest level spells are precious resources, which a player will not use except in a truly tough encounter or dire situation, so that they are able to have these spells available when they **really** need them.
As Mass Heal is currently implemented, it's really only useful as an out-of-combat heal spell (although it is pretty spectacular at doing that)