Okay, there are quite a few issues here.
First off, you cannot use the variable health. it is a built-in, global variable. What this means is that your enemy and the player are both using the exact same health variable. So when the enemy dies, the player dies. You need to make your own variables.
Consequently, the No More Health event only applies to that global health variable. You're going to need to have instead code in the step event which checks if the custom health variable that you created for that particular instance, either the player or the enemy, is less than 1.
kill all instances at relative position (0,0)
I never used 8.1, I skipped over it from GM 8 to studio. But, this looks like you are using the wrong action, assuming that the correct action is still available. What you should be using is just a normal Destroy Instance action. the normal action has no arguments for you to fill in, you just throw it into your code and that's it. It looks like you are using an action such as Destroy At Position, which requires two arguments, x & y. Using the normal Destroy Instance action, you need to change the scope of it. If you look at the top of every drag-and-drop action, there is a set of radio buttons labeled self, other, and object. By default self is always checked. This means that the code action will apply to the instance that is calling that action. The other scope meanns that the code action will apply to the instance that you collide with when use inside of a collision event. Outside of a collision event, the other scope means nothing, but inside a collision event, it will apply to the instance that you are colliding with.
Now this will have a slight drawback which in most cases never amounts to much of anything and realistically shouldn't amount to much of anything, insofar as if a projectile hits multiple targets at the same time, only one target would be destroyed.
Another issue I may see in your code is it appears you are not destroying the fire ball after it hits a Target. this means a fireball could potentially deal damage multiple times to the same Target.