Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
{R}, Sacrifice Heartfire Immolator: It deals damage equal to its power to target creature or planeswalker.
Players can't take any actions between the time you begin to activate Heartfire Immolator's last ability and the time it's sacrificed. Notably, opponents can't try to lower its power.
Use Heartfire Immolator's power as it last existed on the battlefield to determine how much damage its last ability deals.