Hell Spikes does physical damages, and the Survivor's Curse spell can not be resisted (so it is much, much better that the mage's Withering Curse); Kiya liked using Curse + Polymorph on bosses.
Other than that I didn't use many spells, so don't know anything that wasn't already mentioned in the mage guides or other links above. Below is the full skill list, if you want to check which spells can be resisted.
RPG Vault - Divine Divinity Skills XI (links to other skill sets at bottom of page)
