EMP grenades would have the robots short out briefly, but unless they are repeatedly attacked with them, it won't disable them fully. You would also need to find materials in order to make them!

As for acids, just pouring acid on a robot would only slowly eat away its chassis. An acidic attack (magic, projectile, etc) would be more effective, as it would puncture the metal and eat it from the inside. Again, this is dependent on materials found to create such a thing!