I have the same problem.
I made a global variable named EnemyCounter, and also made a global variable named Score.
Every time when an enemy object is created it adds 1 to EnemyCounter, and when an enemy object is destroyed it subtracts 1 from EnemyCounter and adds 30 (for example) points to Score, but when 2 (or maybe more) enemy is destroyed at the same time, it still subtracts 1 from EnemyCounter and adds 30 points to Score instead of subtracting 2 from EnemyCounter and adding 60 points to Score.
Is there any solution?