How to tame an oversized Rex:

1-Tame a Raptor and mount it

2-Get a large amount of narcotics/bio toxins

3-Find a Giga (mostly found in mountains)

3-Sneak around its legs and put metal gateways, you can also put a large trap for it not to easily escape

4-Knock out by using 60-70 toxicant arrows with compound/crossbow or 50-60 tranq darts with longneck

5-Put all the narcotics/bio toxins you have and give it prime meat or kibble

6-If your on mobile give 3x,10x or 15x soothing balm (best option)

7-Feed it the narcotics/bio toxins if its torpor is decreasing

8-You have yourself a monster than can one shot a low level bronto!