Pretty much the giga of scorched earth. Can damage stone, which is nice in pvp. It can throw rocks. This does massive damage to creatures, but does none to structures. Be aware of homing rockets though. If your opponent uses homing rockets, then it doesn't take too much to kill it. If you're not using it actively, you can also disguise it as a rock and put it somewhere around your base. This is a death trap for any raiders coming to attack you!
Rock elementals are extremely tough to kill or knockout. Over encumbering them inside or around your base and setting them to aggressive allows them to defend your base with their rock throw attack without being kited away by enemies.