I play on PVE so I can't use high tech tactics. So far I've tamed about 6 Rexes, 3 of which were lvl 145. (Still looking for the elusive 150)
Anyway the tactic that seems to work the best for me is to kite the Rex to a large shift in terrain. Something the Rex can't walk over. Honestly doesn't have to be much tho, they seem to get stuck on even smaller rock formations.
Once its stuck hit him with a bunch of trans arrows. (You can always break the rocks with a pick axe to get it out once it's tamed.)
I recommend using a scorpion to tame it, pump all levels into health on the scorpion. Then just attack the Rex but with 10 to 20 seconds in between hits, DO NOT pump anything into dm. Also this is what I use to tame literally everything, and it doesn’t even cost anything. Pls put this up in chat for everyone to go “oh that’s not a bad idea” 👌😄👍
Don’t be afraid they are really slow, just get some raw prime, a crossbow, narcotics and some tranqs and go for it. good starter tame you can have follow you until lvl 70 when you can ride it. also very good for bosses if you have ascendant saddles and high levels