How to build a semi competent mage

[ravenfish]

The Radically Unstable Magery limitation, on p.26 of Thaumatology, causes every failed spellcasting roll to be treated as a critical failure, and caps effective skill at 15 to keep these failures from being too rare. It nicely ensures that, succeed your rolls or fail them, spellcasting will never be boring!

EDIT: I just noticed you specified harmlessly blows up in her face. Critical failures with magic are anything but harmless, so the above may not be your best bet. I like Stormcrow's suggestion of using Unreliable to ensure a significant chance of failure irrespective of skill; I might halve the value of Unreliable if a failed roll for that limitation, rather than stopping the ability cold, creates an effect that isn't what was desired but could still potentially be useful.