Hmmmm, that is indeed a tough question... I can think of a way, but it may require PB to do...
There is a condition that PB has called "shotbyweap=X", but if you leave X blank and just use it as "shotbyweap" (exclude the "=X" part) it should return TRUE if the entity has been shot by the player... now, this part I'm not sure about, but it's worth a try! If you put the "shotbyweap" condition in the enemy's death script, it may ONLY return TRUE when the enemy was killed by the player, and not by the AI.
If anyone can give it a try, I'd like to see if it works or not.
The one and only,
Only those who sow the seeds of their desires will reap their benefits later.
However, I have seeds of my own to tend to. I don't have time to be someone else's watering can.