Nice game!
Just one note: I could kill the enemy by leaving the bullet at the bottom. Is it intended?
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No it's actually fine.
And yea, unity physics often bring headache XD
I'm also having problem making it work the way i wanted haha
Or probably you could check for the velocity every time it collide with the enemy before register it as a hit. (the condition for velocity value should be low but you can't use zero. This i guess need a lot of trial and error)