Maybe where magic vs. magic is concerned, it should work like combat. You attack the spell.
Spell AC vs. magic is 15 + 1/2 Spell slot level (or 10+ or 12+, or 8+ spell lot level, or something balanced and enjoyable...).
You're attack is your normal attack DC for the spell casting class.
You cast counterspell, you roll a magic attack vs. the spell AC. If you hit, the spell is countered.
You can give BBEGs special abilities that increase a spell's AC.
IMHO, giving spells HP and having Dispel's do "damage to magic" and have any counter-magic work similar to combat would make the game easier to grok, and add a potentially nail-biting mechanic to a fight.
Edit to add: Maybe just give a +1 to attack for each slot above 3rd it was cast?
Spell AC vs. magic is 15 + 1/2 Spell slot level (or 10+ or 12+, or 8+ spell lot level, or something balanced and enjoyable...).
You're attack is your normal attack DC for the spell casting class.
You cast counterspell, you roll a magic attack vs. the spell AC. If you hit, the spell is countered.
You can give BBEGs special abilities that increase a spell's AC.
IMHO, giving spells HP and having Dispel's do "damage to magic" and have any counter-magic work similar to combat would make the game easier to grok, and add a potentially nail-biting mechanic to a fight.
Edit to add: Maybe just give a +1 to attack for each slot above 3rd it was cast?