I've had a similar problem a while back, and randomly helped me fix the error, but it has returned once again. I tried adding the "For each" condition and "is playing Default" but it ain't working. When I kill one enemy, the others die too. So, what's the problem?
Just looked through your .capx.
The issue doesn't lie in the destroy-event.
It lies in the conditions that subtract the health from the enemies.
Those trigger for every enemy, and that is the problem.
Usually, you choose a "on collision" event. This automatically picks the enemy that has been colliding with the bullet.
But since you are checking for collisions with the hitbox and not the enemy, all enemies are selected and thus, the health is subtracted for every enemy.
This fixes it:
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And once again, randomly saves me. Thanks a lot, man!