Our rexes had 80k HP and 6k% damage which is 2k per bite. We had lost 3 of rexes. Beta Dragon.
The Dragon isn't the only thing you should worry about. The dimorphos will kill you off your mount if you don't have a good armor. It is better to bring a lot of people to mount rexes, as about 2 out of 3 rexes died in lava. They just stand in lava and bite the Dragon. Reaper could be good if you have one(for killing dimorphos), but prepare to lose it - Dragon ignores its natural armor. Good luck, survivors.
Bring Therizinos instead of Rex’s, since herbivores take reduced damage from the fire, and can eat sweet vegetable cakes to regen health quickly. Take about 17-18 theriz, 2 hell pigs, and 1 yuty as well as an OP rocket launcher or shotgun, or substitute 4 theriz for allo’s to get the bleed attack.