Although their movements are dull, no matter how much you attack the mud parts, it has no effect; and if it comes down to a dangerous situation, the skull in its chest hides further inside of its body.
In regards to attack methods, other than attacking people physically with its body it uses magic almost like a rock bullet, as well.
However, the reason why it is included as A rank, isn't that.
Monsters of low intelligence obey its commands.