if they all inherit from a parent class then then you would use the parent class. if not then you just need a way to populate the array with all the actors you want. it really depends on your setup / circumstance. worst case you populate your array with every character in range then do a check to see if the one being tested is a enemy type.
personally when i create enemies i make a base enemy class then make the different enemy types children of the base. this process makes it so you dont need to make multiple instances of things like health systems and it makes it easier when trying to get any kind of enemy.