So my opponent had a Mindshrieker in play and used its ability targeting me. In response, I cast Izzet Charm targeting the Mindshrieker for 2 damage. My opponent argued that the damage from the Izzet Charm wouldn't be calculated until the stack resolved completely, meaning that his Mindshrieker lives no matter what card is flipped over. This seemed odd because if instead of using an ability he had tried to Giant Growth it, and I burned the creature in response, then his creature would die. What's the difference here?