There are several levels of magic in Ogre Battle 64. You can have basic magic, advanced magic, witch magic, and Drakonite magic. What spell your unit will use is based on their class and equipment. How many units your spell will hit will depend on its level and if it has been combined with another speller caster.
Area of Effect:
Depending on the spell the area of effect will change. The spell might target a single unit, while others affect the entire unit equally. Below are all the know area of effect variants, and damage output is approximate.
Single Unit: Direct attack on one unit.
Small Range: Targets a single unit with minor damage to surrounding units.
Corner Target: Variant of the small range.
Wide Range: Targets entire team, with less damage in the corners.
Entire Team: All targets are effected equally.
Ready for some really powerful attacks? It is time to combine your magic! To combine attacks your spell casters must be in the same row and be of a similar level. If your casters aren't combing make sure they are forced to target a similar unit like "Attack Leader". I have also found that the item "Flag of Unity" seems to help as well. Needless to say, after a party has been established for a while, your spell casters will start combining with higher consistency.
Types of Combination:
Basic + Basic: Recommended in the early game.
Basic + Basic Mixed Element: Recommended in the early game.
Advanced + Advanced: Recommended.
Advanced + Advanced Mixed Element: Highly recommended due to the bonus of status effects.
Witch + Witch Mixed Element: Recommended in the early game for status shenanigans.
Basic + Advanced: Not recommended.
There are more combination types out there when you throw in a THIRD spell caster into the mix! Generally adding the third will increase the power of the spells used. If you want to try out mixed elements, two of the spell casters must have the same elemental spell.