To help knock out a Rock Elemental is with a Paracer mobile base with a cannon. Get him trapped on a cliff side aim at his head from high ground and be careful if you are kiteing him to the spot be sure to keep a medium distance from him or else things will go bad. Good Luck! (Bring plenty of Sulfur if you don't have a Mantis Kibble farm.)
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 .