Some great ideas everyone! Hex, not sure the tone of the story, but with my thought about weakening the connective tissues between the rocks (assuming the creature is made up of separate rocks)...if the material holding the rocks together is ground rock dust bound by secreted mucus (I know, I know, why would a rock need mucus? But we are talking about a mobile rock-creature, so why not mucus?
)...what if your MC was carrying a prescription-level congestion-relief nasal spray? He could squeeze the whole bottle into one of Rocky's major joints...it could dissolve the mucus, thus weakening the joint, and the affected area could give out, thus disabling the creature--making it immobile. (I have had a lumbar disc rupture, and believe me, immobility is the only option...well, you do writhe in agonizing pain too, but basically, you're immobile.
).
Once the creature is incapacitated, your hero can fracture/explode/etc the creature at his leisure. This idea might work best if there is humor in your story, though. I hope you'll let us know your final solution to the problem!
CC
ps--okay, here's my last thought!
I guess what I'm saying is that if you can damage one major joint in the creature's body, you can eliminate it as a threat. This way, he wouldn't have to completely destroy the creature...he can walk away, and the creature can even recover (as my back sort of did).