Think even simpler and just use your eyes.
Activate only when a Spellcaster-Type monster declares an attack. Each player Special Summons 1 Spellcaster-Type monster with 2000 or less ATK from their Deck in face-up Attack Position.
Pay 800 Life Points to negate a Special Summon of a monster and the effect of a card that Special Summons a monster and destroy it. Both you and your opponent can use this effect as long as this card remains face-up on the field.
Read the words in bold and go figure.