I know exactly where you are coming from. Many spells are just too inaccurate to contemplate using. I prefer to stick with precision-100 spells.
For Marignon use:
Phoenix Power followed by Incinerate.
For the Grandmasters have someone cast Light of the Northern Star (or use a banner of the northern star) and have them cast Astral Fires.
Both these effects are precision-100 and armor negating. What's not to like?
The other big effect for Marignon is Flaming Arrows, which works well with the crossbows.