When you use on destroyed you are picking only the instance that was destroyed, so you cannot then pick more monsters. You can put those events into a function, then on destroyed : run function. Function allows to pick from all instances again.
what's wrong with my code? when monster is dead, then player will find path to another monster on the field right? again and again.. Do I need "for each" or something?
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UPDATE
this is works so far but got stuck in the end