That's a lot of batteries

There is no way to lower the voltage without wasting power, even the best optimized switching regulators waste a bit of it. You have two ways to solve this, either use a pre-regulator, or use a regulator with wider voltage input. But beware of efficiency, even if your regulator says it has high efficiency, it won't be when regulating with that voltage difference.
I don't really know about your robot, but for example if you need not much power, you could try tapping of from fewer batteries, but only when this is possible and won't discharge these battteries too much. It all depends on what you need.