Instead of lowering the light object's opacity, put the light object on a separate layer* (below your objects for the result you're after judging from your picture) and lower that layer's opacity.
*Don't forget that the default layer in C2 is opaque. If you create a layer and drag it bellow this "layer 0" it will be covered from it. I suggest creating 2 new layers and put your sprite objects on the top layer, your light object (which will hold the shadows) on the middle layer (remember to lower its opacity), and keep the bottom layer for your backgrounds.