I found very useful Water spells. You can block the way of enemies with Ice Wall, or gain some extra turns by freezing them. Occultism spells are great also, I forgot to tell you about Charm, a spell that makes enemies attack one each other. You can't Charm bosses, but it's really helpful.
Water and Occultism are great, imho.
Maybe you can teach Fire spells to the ranger, so you can set things on fire from the distance.
