Oath of Destruction (+50% damage) and Blind are very efficient (de)buffs. Other than that you have Bloodletting for chars with Leech. The Skeletons not only come with poison immunity (fighting against Spiders!), but also do non-elemental damage vs enemies that are immune to magic.
The 2 other spells that I keep using are Death Punch (very high non-elemental damage) and Enfeeble aka Soulsapp - among other things, this mother of all debuff spells gives -50% to all resistances (including non-elemental damage) and drastically reduces the targets saving throws, making it a good boss killer spell, since it stacks with Nullify Resistance.
Later in the game you can pick up Resurrection and Invulnerability, but I can't say I have used these much.
