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.
If you have the resources, tame a titanosaur for the alpha dragon cause the titanosaur is OP and can slay the dragon. (Bring more tames like yuty and daeodons
if you need) Just be cautious about dimorphs in the battle!
Use as many therizinos as you want since it does extra damage to carnivores technically since the therizino is an omnivore which means carnivore and herbivore put together so since the other half is herbivore it should block out that affect I would also recommend pumping damage and health. Thanks for using your time on my tip I hope it helps you beat the boss.