First do some lava caves and get a good ascendant crossbow and then get some tranq arrows (100 is enough) get a high hp mount like baryonyx, carno and maybe a raptor. after that you need to find one and shoot it on your mount once its unconscious (i recommend u to use 10x smoothing balm that u can get by tributing the artifact of the hunter or artifact of the massive) than put some argy kibbles, prime meat or some normal meat and boom the big boy is tamed now no one can stop u (expect gigas lol)