I tamed my Mosasaurus with a Sarcosuchus. The Sarco was very fast and was able to instantly regenerate stamina underwater. I was able to shoot tranquilizer arrows from the Sarco’s back. I also used a SCUBA tank so I didn’t have to surface in order to breathe. The Mosasaurus was level 6 when it was wild. I was on mobile, so I took a soothing balm, gave it to the Mosasaurus, gave it some meat, and used about 100 narcotics. With the soothing balm, it only took about 11 meat to tame. He now sits in the water right by my raft and has to defend himself from sharks every once in a while.