I just found another interesting practice method; enter fake combat, then cast Summon Elemental. Doing this, I noticed 2 things:

1. It counts as practice, even when there are no enemies around. This is especially nice for Alchemists who have a shortage of good practice spells, but it's likely good practice for Mages as well. (Bishops could use it, but note that this spell is in 2 spellbooks, so it only trains the better one.) I was able to get my Ranger's Divine Magic up to 35 this way, allowing her to reliably cast Heal Wounds at power 6. (Power 7 seems to be a lot more demanding than power 6 in my experience, and power 6 is often strong enough.)

2. At power 6 (maybe 5?), any earth elementals summoned will drop Impaling Stones. While it's easier to just buy them, this does provide an alternative source. It works best if you're at the fountain in Trynton that can restore Spell Points (or hex your characters) and has infinite uses.