You can have this emerge from:
"Light" fighter makes multiple attacks and their dodging works as well against spells as against blows
"Heavies" have damage reducing armour that makes the multiple attacks much less effective, but armour protects poorly against magic. Their big attacks can cripple the squishy light when they connect
Mages defend using defensive spells. A spell will stop a given number of attacks, no matter how much damage they do, so they're quite good against the big attacks of the heavy but quickly worn away by the light