ViewportBottom(layer)
ViewportLeft(layer)
ViewportRight(layer)
ViewportTop(layer)
Return the co-ordinates of the viewport in the layout of a given layer, by its name or number. Not all layers have the same viewport if they are parallaxed, scaled or rotated separately.
You can convert the coordinates using those expressions.
enemy x on layer 1 = enemy.X + ViewportLeft(1) - ViewportLeft(0)
enemy y on layer 1 = enemy.Y + ViewportTop(1) - ViewportTop(0)
There's an example in this post that converts coords on one layer to another layer with different parallax.
But... why not have a separate normal layer for the arrows in between the game and GUI layers?