2 ways I can think of.
1. You could add something called a RayCast to the enemy, then every frame check if the player's position is within the RayCast's size, or
2. I would suggest using a CollisionShape2D node to the enemy. Under "shape" parameter use a circle and extend it to the radius you like.
Next find the Inspector panel. Next to it is the "Node" panel. Select it and attach a signal for the AreaEntered() of BodyEntered() signal (depending on if your player is an Area2D or a KinematicBody2D.
When your player enters the CollisionShape of the enemy, it will trigger the signal you created and your enemy can start firing at the player.