Best way to get paint off of a boulder

I've recently discovered a boulder along a beach where I live that looks like it could be made into some great routes. The problem is that the city more than a decade ago painted the entire rock. The paint is really old and and the city has not maintained it (chipping/grimy/completely come off in certain areas). I have permission to remove the paint, but I am not sure what would be best to use. 

For more information, the boulder is granite and not located near any amenities (piping/electricity). Any suggestions or help to clean the paint off would be greatly appreciated.

Pressure washer. 35000 psi or more. Gas powered. 150 gallon water tank in the back of a truck fed by a gas powered pump.

Elbo grease won’t do it, and neither will chemicals. 

Elephant Snot and pressure washer should do the trick.

How do you two expect him to use a pressure washer without water? 

Dan D wrote:

How do you two expect him to use a pressure washer without water? 

No water but he’s along the beach?  How’s that without water?  

Elephant snot should work without any pressure for the water. It just needs to be MILDLY agitated with a brush once when applying and after sitting for a few minutes. Elephant snot is not environmentally harmful but the paint is so have a clean up plan. You don’t want that paint going right back into the water.