im not a SS, but imo, I would only level either MT and BCM. I am not sure about the current content, but it seems it is a lot better to use 1 bullet per splash than stay at a distance to cast MT or BCM (plus both skills is easily affected by terrain, wrong positioning means your bullets will be wasted) I will pick one of the skill since I would still grind normally (DQ) and a mob clearing skill is still needed.
Anyway, here is my GM centric SS build. I think it is hard to abandon the towers and ducks since they pretty much deal good damage with minimal effort (just do the basic planning and deployment) and use splash/ppb ex whenever possible.