I would recommend a Giga trap. Three behemoth Dino gateways and a large bear in the middle,once the rock elemental is in put one more behemoth gateway behind him. Then shoot him in the head with a cannon.
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 .