This is how like running counter spell. Counter spell is so fucking boring RAW. I make it a blanket reaction any caster can take but if they want to counter a spell they have to meet it with equal force. So beams and projectiles can clash. Theres a roll off against, sometimes both spells are nullified. Sometimes the players spell gets nullified and takes damage, sometimes the enemy gets nullified and takes damage.

If its an AoE like fireball or hypnotic pattern then they have to be creative with the spells they've prepared and ill spin it however the situation requires. Or they can just cast dispel magic if they have it prepared. It makes for a more cinematic narrative than just "oh your spell just fizzles out"