Hello, I created a .capx file to try and describe you my way of doing it (Update your software to the lastest version)
What you can do, you can give the enemy an instance vairable, let's call it 'coin', his value will be '0'.
If the bullet is overlapping (or on collision) with the enemy, and the value of the coin is 0, nothing should happen
But, if the coin value is '1' and the bullet is overlapping the enemy, destroy the enemy.
Which means, when we collect the coin, we destroy the coin an change the value of the coin from 0 to 1.
Here's the .capx, hope it helps.
Dropbox Download Link