To ensure it's always working for multiple enemies I use functions. You can pass the UID of the enemy or slime through a function and then using a condition that picks by UID parameter you can apply the actions to that specific instance of enemy or slime. So I would move all spawn actions into a function and test it out, and yes remove the trigger once.
And lastly, how to make it so the damage is not area of effect? What do you mean?