If I understand right, in the image :
Angle(World,Player) = 45
Angle(World,Enemy) = 270
(By the way, usually, angles are handled counter-clockwise)
What you want is Angle(Enemy,Player). Let's decompose angles then :
Angle(Enemy,Player)
= Angle(Enemy,World) + Angle(World,Player)
= - Angle(World,Enemy) + Angle(World,Player)
In the schema's case :
Angle(Enemy,Player) = - 270 + 45 = -225? = 135?, seems about right.
Then, hit location is just a switch case.
If the angle(Enemy,Player) is between -45 and +45, it's the front side.
If the angle(Enemy,Player) is between 45 and 135, it's the right side.
If the angle(Enemy,Player) is between -45 and -135, it's the left side.
If the angle(Enemy,Player) is between 135 and 225, it's the back side.