To knock out a Rock Elemental you can use a Paracer with a plattform sattle equipped. You now could place a ballista turret, a catapult turret or rocket turret on it. However the cannon is the absolute most effective cause it does the most torpor per shot. Not like the catapult with only 5. You also can use a Rocket Launcher wich is also pretty good.
It's a bit silly that kibble tames it a lot better than rocks or sulfur because even in the dossier it says that it lives off of absorbing nearby rocks and minerals. So mabey they should make it so you have to feed them a rare mineral like obsidian what if they made it so you could tame one fast and with high effectiveness by giving one a few hundred or few thousand obsidian . or what if there kibble was made out of a combination of ores and minerals .