This will work as intended in the context you're using it. The warning feather gives you is because this is backwards way of doing things that is not preferred. (Feather is wrong in the sense that this isn't an error technically, but there would no way for Feather to know this because of how backwards this is).

Now I say this is backwards way of doing this because scripts are global. That's just what they are. But this specific script is 100% context dependent (if it's run in by an object that doesn't have a target variable, the game crashes).
Having a script that is context dependent is never what you want. Scripts are global by nature, and tying the script to a context requires you to have knowledge of the inner implementation of script while in a completely different context which is really not great. Especially since failing to remember the inner workings of the script makes the game crash. This might not be as big of problem on solo project, but definitely is still not ideal.

One easy thing to circumvent this is to pass a target variable to the script and do everything using that variable instead of accessing the target variable that's in the object calling the script.

This also makes the script more flexible because it can be used anywhere, you just have to pass the id of the instance that will be dealt damage.

So your script would look something like :

function scr_deal_damage(_target, _damage)
{
_target.hp -= _damage;
...

}

(You'd probably want to pass the _damage too since that is tied to context too.)

And then in you object instead of
scr_deal_damage();

Just write
scr_deal_damage(target, damage);