In my example best way is first create shadow map for all entites in level, and after collopse this shadow map with your terrain colormap. This looks very nice in my game and i dont lose much time for this.
For generate shadow map you can use some code for this. I think Loopix made some in mysty mode. After when i generate shadow map i copy he in my color map wich use multi shader script. For this you can use PhotoShop, Gimp or something else. At the end we have color map with shadows for your tress, buildings, houses... Also you can paint in PhotoShop how shadows color be strong.
I dont kow any better solutinos for cast shadows on terrain in 3dgs :-)
