The issue here lies with the way you are adding and removing the enemies from the “_targets” element. Since both archers can target the same Enemy, that creates the issue. Once one archer Destroys the Enemy, the second archer cannot access to Destroy the Enemy since the value is already null in the list.
The fix to this could be that both archers have different lists of targets and couldn’t target an enemy that is already in the other archers list of targets. Another suggestion could be that you remove the specific Enemy that was hit, and not the first entry in the “_targets” list.