The System expression angle is what you are looking for. You pass it two sets of coordinates, a starting position and an ending position, and it will calculate the angle between the two points.
In the following example project I use it to set the angle of instances using the Bullet behaviour. The bullets are created at the position of the player instance and then an angle is calculated between that position and the mouse pointer position.
You should be able to accommodate this idea to what you are working on.