from the moveTowards tooltip :
The Target can be specified as a Game Object or a world Position. If you specify both, then the Position is used as a local offset from the Object's Position.
so in your case, you would need some small rig:
have a "target" dummy gameObject that follows the position of your ennemy you are shooting at, this "target" is looking back at the player firing. the arrow is moving towards the "target" with a local offset position as described in the tooltip.
You can't use the ennemy gameObject for a local offset, because he could be in any orientation, that's why you need to first set up this "target" so that the arrow continues its path in a realistic manner.