I seem to remember a thread on 'apprentice level spells'; I will need to search for it -- but they collected quite a few. There is also an online resource / spell reference for original spells of all levels.
I would suggest Demon's Eternal Oblivion at level 5. Even at that level, it will destroy a Might of 13.
PeVi Base 3 (reduce Might of target by 3+10); R: Voice (+2); Duration: Momentary (+0); Target: Individual (+0)
Handy for an apprentice to defend themselves vs minor Infernal pests, and it will be useful even against more powerful foes as the Magus advances -- because the penetration total will steadily climb. An apprentice might usefully 'Experiment' with this -- an 'Enhanced Effect' version would be valuable to everyone.
Here are a few resources that I hope will be helpful: