I'm an ex-pipeliner. Done miles of ditch through all kinds of rock. Dynamite is the tool of choice but occasionally not usable due to location of houses, foundations, high pressure gas lines etc. In the absence of an excavator mounted breaker the best tool is a "90 pound pavement breaker" with a bull point (also called a bull_ _ _ _) bit. The bullpoint bit has a sharpened point much like a nail. Simply chip off a piece at a time. Hard work but gets it done. The breaker requires a large wheel mounted air compressor, usually Diesel powered. Both are available at rental yards. Wear your ear plugs, safety glasses and hard toe boots. If they didn't work they would've stopped making them years ago.
When people refer to "Jackhammers" there is often confusion as to exactly what type hammer they are referencing. There are 2 types of pneumatic hammers. One type called a rock drill, turns as it impacts. This is used to drill holes for pre-splitting or loading of explosives. The other type is a breaker which impacts with no rotaion. This is used for breaking rock, or when equiped with a flat blade that looks like a shovel, cutting asphalt or concrete.