The effect "Once per turn, during your Standby Phase, if your opponent controls more monsters than you do, you can destroy a number of cards your opponent controls so that your opponent will control the same number of cards as you control monsters" makes a Chain Link.
If the card "Core Blast" is activated during the Standby Phase, its effect can also be activated in the same Chain Link as the card.
The effect of "Core Blast" does not target. It destroys Monster, Spell, and Trap Cards on the opponent's field so that the number of Monster, Spell, and Trap Cards on the opponent's field is the same as the number of monsters in the player's Monster Zones. (The cards that are destroyed are chosen by the player who activated the effect of "Core Blast".)