To add to most on here, the best way I've found is the cliff method but just in case you don't have a cliff take 2 metal billboards and place them in av shape using a tame with high hp find your rex of choice, coax it to the v, then tranq it with your tame behind you, once it's in the v use your tame as a door for the opening of the v.

Start the tranq process and knock it out, once knocked out use exceptional kibble for best results, keeps tame percentage at 100 and you get the extra lvl you need for it, yes they're tanks but with the rise of the giant bugs, they can possibly take down 1 low lvl but not 2. I was lucky to survive a mid lvl bug and a theri aka tickle chicken, also one the rex is down so reference the starve timer on here with the kibble for somewhat faster taming