Spell-caster vs. Spell-caster:
When a spell is cast at another who possesses the same spell, the higher level caster will have a magic immunity of 3% per level against his or her opponent.
Example:
If a 17th level character casts a magician spell at a 26th level magician, who possesses the same spell, the higher level caster will have a 9% chance of not being effected by the spell.
The level difference x 3 = the % chance to not be effected by a lower level spell-caster's magic (if you have the same spell).
This applies to all types of magic, BUT THE HIGHER LEVEL CASTER MUST HAVE THE SAME SPELL.