Easiest way to tame is make a rocket launcher and only take head shots. Body shot do way more damage and will likely kill them. Also best strategy is find a cliff with two sides and have one player on each side shot rockets at him. Then feed mantis kibble or sulfor.you will need about 20+ rockets for a low level one.
For some reason, after the latest update, X-rock elementals (haven't checked other variants) are no longer tameable. Whenever you knock them out, they don't eat and don't have a taming bar. Forcetame also doesn't affect them, so I believe it's a glitch