In general:Faren spells gain spellpower in nature areas. Specifically in areas under the open sky.
Specific Spell Improvement tips:
Shatter: Shatter can be improved even when cast on a person with no items in their inventory.
Earthquake: Earthquake can only be cast with targets in the room. For that reason many people like to work this spell in Badlands II, where bramble walls regenerate themselves and therefore there are nearly constant targets.
The other approach is to simply use it as a tool as you build HP and farm shillings. It is very useful to pull mobs for wall spells in OPBL, OOK, Brax, Ukgoth or the Bone Preistess maps. It also helps greatly to clear skeletons from Marion Crypt 2 either when farming statues for HP and roses or when clearing statues for access to the third level (the mobs in Marion Crypt 2 also drop a nice ammount of red and purple mushrooms).