Situation: My opponent has a Zombie Army with 3 +1/+1 counters, and also a another Zombie Army (say for example, Amoeboid Changeling). They cast a card with Amass (for example: Relentless Advance). I have a Jaya's Greeting in hand.
Question: Can I choose to respond to the Relentless Advance by destroying whichever creature will get the tokens? Or does my opponent choose after it resolves?