Heres 10 easy steps to tame a Bronto if ur lvl 21+ :

1. Tame a Ichthyosaurus

2. Put a saddle on it

3. Get about 3 bows (they break easily)

4. Make a TON of tranq arrows (60-100)

5. Go to herbivore island on your Ichthyosaurus

6. Find a Bronto (Stay in the water, and keep a distance, it wont agro)

7. Get it unconscious

8. Make sure you have a TON of berries and some narcotics for it so it doesn't wake up (i recommend having a Trike/ Stego for berries)

9. Feed the berries to it and keep narcotics close if its close to waking up!

10. Enjoy your new Bronto!

I hope this helped!