Okay please bear with me because it has been a while since i last used 2003, and it doesn't seem to want to install on my computer...
Make a battle event that activates when the monster uses the attack. All of the code should be in the little tabs at the bottom, it's like editing an event. Make a new variable, set it to the amount of remaining amount of health the hero has, subtract one from it and then damage the hero by the amount in the variable. If you have more than one character in the party, you could either reuse the same variable repeatedly for every character, or just make one variable for each character.
That's all i gots.