I'm considering a system of magic that allows for multiple ways of evading harm.

1. Will~ Using your force of will and magical training, you can put up a defensive ward that is instinctive and simply moves the enemy spell around you, or makes it vanish entirely. This is the fluffy explanation of Magic Defense.

2. Agility~ Spells that take physical form, such as fireballs, can be evaded, blocked, dodged, etc.

3. Counter Spell~ Casting a spell that acts as a barrier or shield to absorb the damage instead of you.

The problem:
How to write it in. Do i segregate the defense types by their Magic School, or should it be treated on a spell-by-spell basis? Also, should it be done at all? Should i leave Will as the only magic defense and say Agility be left to physical defenses? Should counter spells be used only on your turn? Or perhaps make Counter Spell a skill that can be learned, like a Riposte?