This is a really unique idea, I like it a lot! I had to play it a couple of times before I was able to memorise the shield and heal commands, but I wasn't able to remember the buff command, my short term memory just isn't good enough!
I wish I could have a bit more control over the small shield though, I'm pretty sure it's impossible to avoid ALL damage unless you use the spell. While that seems like an apt metaphor for the healer being blamed for not accomplishing an impossible task, it was a little frustrating to actually play.
Perhaps some easier enemies to start with, so there are little moment of victory along the way to harder enemies? Oh, and you could have easier spells too to start with! I think it's a really creative idea, I just feel it needs to ramp up a bit more slowly.
Great job!
(-Joe)